GEOSAT Follow-On (GFO) Altimeter Document Series

NASA/TM--2001-209984/VER1/VOL. 1

r Volume 1 GFO Altimeter Engineering Assessment Report

From Launch to Acceptance 10 February 1998 to 29 November 2000

i" Version 1.0

David W. Hancock III George S. Hayne Ronald L. Brooks g.. Dennis W. Lockwood

w March 2001 i

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iF NASA/TM--2001-209984/VER1/VOL. 1

GEOSAT Follow-On (GFO) Altimeter Document Series

Volume 1 GFO Altimeter Engineering Assessment Report

From Launch to Acceptance 10 February 1998 to 29 November 2000

Version 1.0

David W. Hancock III George S. Hayne Observational Science Branch, Laboratory for Hydrospheric Plvcesses NASA GSFC Wallops Flight Facility, Wallops Island, Virginia 2333 7

Ronald L. Brooks Dennis W. Lockwood Raytheon ITSS NASA GSFC Wallops Flight Facility, Wallops Island, _qrginia 2333 7

National Aeronautics and Space Administration

Goddard Space Flight Center Greenbelt, Maryland 20771

March 2001 D

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Available from:

NASA Center for AeroSpace Information National Technical Information Service 7121 Standard Drive 5285 Port Royal Road m Hanover, MD 2 i 076-1320 Springfield, VA 22161 Price Code: A 17 Price Code: A 10 m m

II Foreword

The Navy's Geosat Follow-On (GFO) Mission, launched on February 10, 1998, is the latest in a series of altimetric satellites which include Seasat, Geosat, ERS-1, and TOPEX/POSEIDON (T/P). Data and results from these missions lead to vast improvements in our knowledge of ocean circulation, ice sheet topography, and climate change. In order to capture the maximum amount of information from the altimetric data, accurate altimeter calibrations are required for the GFO civilian data set which NOAA will produce. NASA/Goddard Space Flight Center/Wallops Flight Facility (GSFC/WFF) has provided these products for the Geosat and T/P missions and is doing the same for GFO.

Wallops' multiple roles with regard to GFO are:

• NASA Representative for Radar Altimeter Performance • Calibration Collaboration

• Member of GFO Cal-Val Team

• Data distribution to members of Cal-Val Team

• Validate sensor-related corrections

• Provide corrections for sensor changes

For the latest updates on the performance of the GFO Radar Altimeter, and for accessing many of our reports, readers are encouraged to contact our WFF/GFO Home Page at http://gfo.wff.nasa.gov/

This WFF GFO Altimeter Engineering Assessment Report has been prepared by Ray- theon/ITSS under Contract NAS5-00181 with the NASA Goddard Space Flight Cen- ter, Greenbelt, Maryland. This work was performed under the direction of David W. r Hancock, III, WFF GFO Altimeter Verification Manager, Observational Science Branch, Laboratory for Hydrospheric Processes, NASA Goddard Space Flight Center, L Wallops Flight Facility, Wallops Island, Virginia, who may be contacted at (757) 824- 1238, [email protected] (e-mail), or (757) 824-1036 (fax).

December 2000 Page iii From Launch to Acceptance GFO Altimeter Engineering Assessment Report Foreword --

m 1

From Launch to Acceptance Page iv December 2000 Acknowledgments

This document includes contributions by the following members of the Wallops Flight Facility GFO Team:

• David Hancock (NASA GSFC/WFF) WFF GFO Altimeter Verification Manager

• George Hayne (NASA GSFC/WFF)

• Anita Brenner (Raytheon ITSS/Greenbelt)

• Lisa Brittingham (Raytheon ITSS)

• Ronald Brooks (Raytheon ITSS)

• Jeffrey Lee (Raytheon ITSS)

• Dennis Lockwood (Raytheon ITSS)

• Carol Purdy (Raytheon ITSS)

• Ngan Tran (Raytheon ITSS)

December 2000 Page v From Launch to Acceptance GFO Altimeter Engineering Assessment Report Acknowledgments --

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From Launch to Acceptance Page vi December 2000

I Table of Contents

Foreword ...... iii

Acknowledgments ...... v Table of Contents ...... vii

List of Figures ...... ix List of Tables ...... xi Section 1 Introduction

1.1 Identification of Document ...... 1-1 1.2 Definition of a GFO Cycle ...... 1-1 1.3 Data Flow to/from Wallops ...... 1-1 Section 2 On-Orbit Instrument Performance

2.1 Internal Calibrations ...... 2-1 2.2 GFO Cycle (17.05 days) Summaries ...... 2-10 2.3 GFO Key Events ...... 2-12 Section 3 Assessment of Instrument Performance

Y 3.1 Range Measurement Noise ...... 3-1 3.2 GFO Sigma Naught Comparison with TOPEX ...... 3-3 3.3 GFO SWH Comparison of GFO and TOPEX ...... 3-5 3.4 Wind Speeds ...... 3-6 3.5 GFO Sigma0 and SWH Calibration Correction ...... 3-8 3.6 Waveform Fitting in GFO Data ...... 3-9 3.7 GFO "Smile Patch" and its Consequences ...... 3-21 3.8 GFO Range and SWH Consequences of Thermal Change ..... 3-25 3.9 Sigma0 Blooms and Examples in GFO Data ...... 3-28 Section 4 WFF's Processing Recommendations to GFO Project

4.1 Temperature Correction for AGC ...... 4-1 4.2 WFF Recommended Sigma0 and SWH Corrections ...... 4-3 Section 5 Engineering Assessment Synopsis 5.1 Performance Overview ...... 5-1 fc,r 5.2 Open Issues ...... 5-1 Section 6 References

6.1 Supporting Documentation ...... 6-1 Appendix A GFO Altimeter Sigma0 and SWH Calibration Correc- tion

Abbreviations & Acronyms ...... AB-1

December 2000 Page vii From Launch to Acceptance GFO Altimeter Engineering Assessment Report Table of Contents _,

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From Launch to Acceptance Page viii December 2000

W List of Figures

Figure 2-1 Launch-to-Date Cal 1 Range/Temperature ...... 2-2

Figure 2-2 Period 1999-180 to 1999-229 Cal 1 Range/Temperature ...... 2-3

Figure 2-3 Period 2000-122 to 2000-190 Cal 1 Range/Temperature ...... 2-4

Figure 2-4 Period 2000-243 to 2000-277 Cal 1 Range/Temperature ...... 2-4

Figure 2-5 Launch-to-Date Cal 1 AGC ...... 2-5 Launch-to-Date Cal 2 AGC ...... 2-6 = Figure 2-6

Figure 2-7 Period 1999-180 to 1999-229 Cal I AGC ...... 2-7

Figure 2-8 Period 1999-180 to 1999-229 CaI 2 AGC ...... 2-7

Figure 2-9 Period 2000-122 to 2000-190 CaI 1 AGC ...... 2-8

Figure 2-10 Period 2000-122 to 2000-190 Cal 2 AGC ...... 2-8

Figure 2-11 Period 2000-243 to 2000-277 Cal 1 AGC ...... 2-9

Figure 2-12 Period 2000-243 to 2000-277 Cal 2 AGC ...... 2-9

Figure 3-1 Noise in Altimetry - Spectra Derived by Differencing Repeat Tracks ...... , ...... 3-2

Figure 3-2 GFO Noise Level ...... 3-2

Figure 3-3 Comparison of GFO and TOPEX Sigma0 ...... 3-4

Figure 3-4 Comparison of GFO and TOPEX SWH ...... 3-6

Figure 3-5 Distribution of Open Ocean Collected Data ...... 3-7

Figure 3-6 Comparison Between GFO and QSCAT Wind Speeds ...... 3-7

Figure 3-7 Standard Deviation Comparison Between GFO and QSCAT Wind Speeds ...... 3-8

w Figure 3-8 GFO Waveform Data from Calibration Mode 1 Fitted to Ideal Sinc^2 with 3.125 ns Width ...... 3-12

Figure 3-9 GFO Waveform Data from Calibration Mode 2 ...... 3-13

Figure 3-10 GFO Waveform Sample Data and Model Waveform Fits ...... 3-14

Figure 3-11 GFO Waveform Fit Comparisons of Attitude/SWH Correction to Range for Gate Index 1 and 2 ...... 3-15

Figure 3-12 GFO Waveform Fit Comparisons of Attitude/SWH v Correction to Range for Gate Index 3 and 4 ...... 3-16

Figure 3-13 GFO Attitude Diff. (Fit - SDR) for All Gate Index (1-4) ...... 3-17

Figure 3-14 GFO Waveform Fit Comparisons of SWH Estimates for Gate Index 1 and 2 ...... 3-18

December 2000 Page ix From Launch to Acceptance

= GFOAltimeterEngineeringAssessmentReport Listof Figures _,_

Figure 3-15 GFO Waveform Fit Comparisons of SWH Estimates for Gate Index 3 and 4 ...... 3-19

Figure 3-16 GFO Noise Baseline Examples ...... 3-20

Figure 3-17 GFO Waveform "Smile Patch" Values ...... 3-22

Figure 3-18 GFO Range Correction Error vs. SWH, With NO Patch ...... 3-23 m Figure 3-19 GFO SWH Error vs. SWH, With NO Patch ...... 3-24

Figure 3-20 GFO Individual Waveform Sample Gains from Cal-2 with Patch, at Temperatures 30 and 45C ...... 3-25

Figure 3-21 GFO Range Correction Difference vs. SWH Comparing 45C to 30C Cal-2 Data ...... 3-26

Figure 3-22 GFO SWH Correction Difference vs. SWH Comparing 45C to 30C Cal-2 Data ...... 3-27 m Figure 3-23 NGDR Sigma0 vs. Time ...... 3-28

Figure 3-24 NGDR Wind Speed vs. time ...... 3-29

Figure 3-25 GFO 1-Sec Waveform Fit SWH vs. Time ...... 3-29

Figure 3-26 NGDR Vatt vs. Time ...... 3-30

Figure 3-27 GFO 1-Sec Waveform Fit Attitude vs. Time ...... 3-31

Figure 3-28 SSHU STD vs. Time for GFO Data Segment B, in 2000 Day 252 ...... 3-31

Figure 3-29 NGDR SSHU STD Standard Deviation vs. Time ...... 3-32

Figure 3-30 NGDR Vatt Standard Deviation vs. Time ...... 3-33 W

Figure 4-1 Temperature Correction Analysis Plots ...... 4-2 m L

m J k w

From Launch to Acceptance Page x December 2000 List of Tables

Table 2-1 Cycle Summaries ...... 2-11

Table 2-2 GFO Key Events ...... 2-12

Table 3-1 Comparison of GFO and TOPEX (Ku) Sigma0 Means for 10-Day TOPEX Cycles ...... 3-3

Table 3-2 Comparison of GFO and TOPEX (Ku) SWH Means for 10-Day TOPEX Cycles ...... 3-5

Table 3-3 GFO "Smile Patch" Values for Waveform Samples I - 128 ..... 3-21

T v

L x

December 2000 Page xi From Launch to Acceptance

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December 2000 From Launch to Acceptance Page xii m

I Section 1 Introduction

1.1 Identification of Document

The purpose of this document is to present and document performance analyses and results for the Geosat Follow_on (GFO) altimeter. It is the first of an anticipated series of Wallops GFO performance documents, each of which will update WFF's assessment results. This report covers the altimeter performance from launch on February 10, 1998 to acceptance on November 29, 2000.

1.2 Definition of a GFO Cycle

Like its predecessor, GEOSAT, the GFO groundtrack has a repeat (+/-1 km) period of 17.05 days. For our analyses, the repeat periods are referred to as cycles, and are used as data dividers to assess sensor internal consistency, taking into account seasonal differences. Cycle numbers have not been assigned at this time.

1.3 Data Flow to/from Wallops

1.3,1 To Wallops

The daily near-real time GFO data flow from the Naval Oceanographic Office (NAVO), Altimetry Data Fusion Center (ADFC), Stennis Space Center, Bay St. Louis, MS to Wallops Flight Facility (WFF) consists of:

• Science data without waveforms (ra_data)

• Science data with waveforms (ra_cal_data)

• Engineering data (eng_data)

• Water Vapor Radiometer data (wvr_data) • Sensor data (sdr)

Additional data are forwarded by the Navy to Wallops as soon as the data are available, consisting of:

• Navy Geophysical Data (ngdr)

• Operational Orbital Determination data (oodd)

1.3.2 From Wallops to Cal/Val Team Members

Wallops forwards the following GFO data types to the other members of the CaI/Val v Team:

Sensor data (sdr)

Science data with waveforms (ra_cat_data)

Operational Orbital Determination data (oodd)

December 2000 Page 1-1 From Launch to Acceptance GFO Altimeter Engineering Assessment Report Introduction -,--

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From Launch to Acceptance Page 1-2 December 2000

i Section 2 On-Orbit Instrument Performance

As of November 29, 2000, the GFO altimeter had acquired a cumulative total of approximately 700 days of data out of a possible 935 days. During the initial year- and-a-half of the GFO on-orbit mission, altimeter data collection was sporadic due to various spacecraft systems and software problems, the descriptions of which are out- side the scope of this report. The following subsections will illustrate that the altimeter tracking data have been internally consistent. The subsections discuss:

• internal calibrations

• cycle summaries

• key events

2.1 Internal Calibrations

The GFO's internal calibration mode has two submodes, designated CAL-1 and CAL-2. CAL-1 is designed to detect changes in the internal path delays, to measure range drift. CAL-1 also monitors changes in the receiver automatic change control (AGC); the altimeter's estimates of the ocean surface radar backscattering cross-section are obtained from the AGC values. The purpose of the second mode, CAL-2, is to characterize the response of the receiver and digital filter bank.

During CAL-1, a portion of the transmitter output is fed back to the receiver through a digitally controlled calibration attenuator and a delay line, whereupon the altimeter

L acquires and tracks the signal. Then, during CAL-2, the altimeter processes receiver thermal noise with no transmitted signal present, to characterize the waveform sampler response.

Generally, the GFO Project provides at least two and sometimes three internal calibrations per day.

Prior to Wallops' receiving the calibration data, the GFO ground data processing system routinely does the following: (1) adds a large constant bias to the CAL-1 range, such that the magnitude of the resultant range sum is comparable to a nominal nadir altimeter range to the surface of the earth, and then (2) applies an oscillator drift correction to the total range.

To reconstruct a meaningful CAL-1 range, Wallops performs the following: (1) using the GFO-Project-provided VTCW (Vehicle Time Code Word), removes the oscillator drift correction, and then (2) removes a large constant bias.

The first series of plots which follow contains all the launch-to-date calibrations. These are followed, in turn, by the calibration plots for specific, smaller, cal/val periods of time. The cal/val periods (YYYY-DDD) are: 1999-180 to 1999-229; 2000-122 to 2000-190; and 2000-243 to 2000-277.

= - December 2000 Page 2-1 From Launch to Acceptance GFO Altimeter Engineering Assessment Report On-Orbit Instrument Performance ,,-

2.1.1 Range 2.1.1.1 Launch-to-Date 1998-133 to 2000-334

The launch-to-date CAL-1 range calibrations are shown in the middle of Figure 2-1, denoted by (+) and are referenced to the left vertical scale in millimeters. The data plotted nearer the bottom of the figure, denoted by (,), are the Composite Tempera- tures corresponding to the times of the calibrations; the temperatures are referenced to the right vertical scale in degrees Centigrade.

gfo.wfc98133-00334 • CALl

320 I I I I I 1 I I I I 1 80

300 70 D

O w 28o ÷ 41_ _1,4. ,I, m. •114. _- E E

.__o_260

r U

240 m

220 I I I • I I I i I it30 200 400 600 800 U Days, from 1998-133T05:27:58.01 to 2000-334T23:55:23.27

' Figure 2-1 Launch-to-Date Cal 1 Range/Temperature W

An apparent range calibration drift is observed during the first -400 days (bottom scale). This is not a real drift, however; it only reflects the fact that we do not have the w VTCWs for this early time period.

Subsequent to the time that we began receiving the VTCWs (via tile SDRs), tile range m calibrations have stayed within a narrow window of five mm (273-278 mm). When J correlating the range calibration measurements with the composite temperatures, it is apparent that the range measurements have a minor temperature dependency of i U -0.55 mm/deg, and the narrow window could be reduced even further by applying a temperature correction, if desired. The internal consistency is already within such a

From Launch to Acceptance Page 2-2 December 2000 On-OrbitInstrumentPerformance GFOAltimeterEngineeringAssessmentReport

small window, however, that a temperature correction is not warranted. There is no evidence of any significant long-term range drift. 2.1.1.2 Cal Val Periods

The CAL-1 range for several of the cal/val periods are shown in Figure 2-2, Figure 2-3, and Figure 2-4 respectively. The Composite Temperatures are shown at the bottom of each figure, denoted by (*) and referenced to the right vertical scale in degrees Centigrade. The CAL-1 range is denoted by (+) and referenced to the left vertical scale in millimeters. Each range is in a very narrow band and displays some temperature dependency. 2.1.1.2.1 Period 1999-180 to 1999-229

gfo.wfc99180-99229 : CALl 8O

300 f L 7O

280 60_ + + ++ +-H- ++ + ++4- ÷ +-I_ _-_ ++++++++÷++++++<'_ "-- + ++++-I-++_-++++ +++++++_H4+++++++++++++++ ++++++

- E

'_ 260 -

i O_OOOOO _'o_'_'_' 40 240 ! I O i F- 0ooO0OoOOOoooOoOO 0o°o°°°

t-- 220|1__ 4 I I I I I I J 3o = 0 10 20 30 4o Days. from I999-I80T01:08:49,09 to 1999-229T23:59:31.00 v Figure 2-2 Period 1999-180 to 1999-229 Cal 1 Range/Temperature

December 2000 Page 2-3 From Launch to Acceptance GFOAltimeterEngineeringAssessmentReport On-OrbitInstrumentPerformance .--,

2.1.1.2.2 Period 2000-122 to 2000-190 g

gfo.wfc00122-00190 : CALl 8O i 320_F7 V _ F i T [ i ; r i i f_F F

0 w 300 i E ! -1 F !°0} I ++ +_._++_._._%_ -_--_*+_ ++._+ + +* ++++_ +,++ ÷

4 ¢

260

2,0 i _®o_==_,_= o_ ___ I L o_,_=_=_=°_'_®° _ i ° _

220__L__ Lj_!.llJ{ I I t t I : I I I I I t I J I I I I t I I I I I : I _ I _ t I it i [J __L_L_ 30

0 10 20 30 40 50 60 Days from 2000-122T05:18:01 01 10 2000-190T23:5852 29 l

Figure 2-3 Period 2000-122 to 2000-190 Cal 1 Range/Temperature .=_

2.1.1.2.3 Period 2000-243 to 2000-277 m

gfo.wfc00243-00277 : CALl i = 1 ' i_B0

320 _ _ t

l U

3OO

-++÷++++++++++++++++++++ +++++ +++++++++:+++++++÷++i++++++++++÷÷+ U

_, 260 ......

2,=0 ! i q l ! i

o O O O O ooOoOoOoO 0 0 0 0 0 0 0 I:' 0 0 0 0 0 0 0_ 0 0 i _t 0 0 0 O .0i . otl, O °oOo o i 0 • 0 • i0 0 0 0 0 0 0 • ° 0 0 0 O, N 220' j.= u _ :: i _ _ i _ _ l _ i _ • _ t i _ i i _ ! _ • , 30 n 0 5 10 15 20 25 30 Days, fro m 2000-243T00:04:16 42 to 2000-2773-23:55: t 0 70

Figure 2-4 Period 2000-243 to 2000-277 Cal 1 Range/Temperature m

From Launch to Acceptance Page 2-4 December 2000 I J On-OrbitInstrumentPerformance GFOAltimeterEngineeringAssessmentReport

2.1.2 AGC

2.1.2.1 Launch to Date 1998-133 to 2000-334

The launch-to-date CAL-1 AGCs are shown in Figure 2-5.The AGCs have been routinely temperature-corrected at the GFO processing center using an algorithm (see Section 4.1) derived by Wallops, and have remained in a fairly narrow band of 42.64 + 0.12 dB. Some minor AGC drift is noted, but no further temperature dependency is indicated.

gfo.wfc98133-00334 : CALl 44.0 I " I I i I I I I I I I

43.5 ...... r ...... T ......

O (5 <

"5 43.0 8 -- _ + -

F-

42.5 ...... L...... "{:'- - '- _ ......

= 42.0 I I i ._'__._l I I • l I I 1 ', __L ___ V 200 400 600 800 Days, from 1998-133T05:27:58.01 to 2000-334T23:55:23.27

Figure 2-5 Launch-to-Date Cal 1 AGC

December 2000 Page 2-5 From Launch to Acceptance GFOAltimeterEngineeringAssessmentReport On-OrbitInstrumentPerformance --

The launch-to-date CAL-2 AGCs are shown in Figure 2-6. A CAL-2 residual temperature dependency of -0.06 dB/deg is evident, although the same processing correction applied in CAL-1 has been applied to the CAL-2 data. The CAL-1 mode is more like the actual tracking mode AGC. The WFF team opinion is that the applied correction is more like a normal system correction that includes the CAL-2 noise. Transmit- ters and other corrections when combined are different than just CAL-2.

gfo.wfc98133-00334 : CAL2 230 l r _ _ _ t t _ _ _ J J t

22.5 ...... r ...... r ...... T......

m._

O B g <

_ 22,0 : ! + ', t,.) w

I--

i+ ! + ' qmp 21! :il: ++ : + $ ...... _ ...... _ ...... _ ...... + + =1: m

I 21 +0 I ] I .1. I 1 t I I , 1 I I I I 0 200 400 600 800 Days, from I998-I33T05:27:58,01 to 2000-334T23:55:23,27

l a Figure 2-6 Launch-to-Date Cal 2 AGC

2.1.2.2 CaI-Val Periods i

The CAL- 1 AGC and the CAL-2 AGC for each cal/val periods are shown in Figures 2-7, Fig- ure 2-8, Figure 2-9, Figure 2-10, Figure 2-11 and Figure 2-12, respectively. Similar to the m launch-to-date calibration data, the CAL-I AGC are each in very narrow bands; and the CAL- 2 AGC each display some temperature dependency. R j

u ¸

m • From Launch to Acceptance Page 2-6 December 2000 R

U On-Orbit Instrument PerfOrmance GFO Altimeter Engineering Assessment Report

2.1.2.2.1 Period 1999-180 to 1999-229

gfo wfc99180-99229 :CALI 44 0_ i i i i i b i d i ,, i i i i _ i i i i : _ i i b i i _ i • i i i L _ i i i I : t i i I _ _ i _ i

i : :

435 ...... _ ......

+ L +++++_'+++++++++_++ +++++++_,+++++ ++++++++++++÷++ ++÷++++++ +++++÷ ÷ +++++++++++++ + i i i +

420ft

0 10 20 30 40 Days, from 1999-180T01:08 49 09 to 1999_229T23 59:31 00

Figure 2-7 Period 1999-180 to 1999-229 Cal 1 AGO

gfo.wfc99180-99229 : CAL2 v ...... i...... i......

22.o_ ...... ]...... !...... ;...... _......

+ ÷ + -pt- ++++4-++ + + + : ._ ! + + + ++++++ +++ _,.5 - - i ...... L.'_._.+-_+-*_-._t_____...... :__t____i_.._._*_......

+ ++4- + 4- + +

+ + ÷ + +÷+ +.+++ +++++

210 J .L 1 0 I 0 20 30 40

Days, lrom 1999-180T01:08:49091o 1999-229T23:59:31 00

Figure 2-8 Period 1999-180 to 1999-229 Cal 2 AGC

December 2000 Page 2-7 From Launch to Acceptance GFO Altimeter Engineering Assessment Report On-Orbit Instrument Performance --

2.1.2.2.2 Period 2000-122 to 2000-190. m

gfo wfc00122-00190 : CALl 44.01"I _ _ i t _ t I I . I J i I i i I i _ 11_Ifll il I ! I II_ I l i I I i i I I : I I _ i;l'l:'i:E i=_i i:_:i f I _ li .g

43.0 ...... ! ...... _ i -

* i ! I

i '25i...... i...... i

i :

z i 420]lililllilillillllll lllllllll;JJillll II !JllJll]ll;llllll III ll]lllIIi 0 I 0 20 30 40 50 60 Days, from 2000÷122T05:18:01.01 to 2000-190T23:58:52 29

Figure 2-9 Period 2000-122 to 2000-190 Cal 1 AGC

gfo.wfc00122-00190 : CAL2 23.0 k : _ , J"_ k • I I I i i I _ t k _

m I

22.5

a 1 i 22.0 ...... "...... ] W

2,5 -+ + .÷'.. +• +++_÷_*_* + ,+_- ++ 4+ ++÷

++_*+÷:* + . : :

21.OJ i _ _JJ {!_- _ _ [_LI [ J ' I I J I _l kJ_k' [ i J i 1 t i 11 " i I I I J II1_ 'liIIllili'lllllllll m 0 10 20 30 40 ,50 60 Days, from 2000-122T05:18.01 01 to 2000-190T23.58:52.29 m

Figure 2-10 Period 2000-122 to 2000-190 Cal 2 AGC

From Launch to Acceptance Page 2-8 December 2000 .._ On-Orbit Instrument Performance GFO Altimeter Engineering Assessment Report

2.1.2.2.3 Period 2000-243 to 2000-277

gfo.wfc00243-0027"7 : CALl

4401 I t 1 t , _ r _ i ,, I I I I I I I I • I I I I , I t I I - t 'T t

F _ P

+ - _ *_.oI......

+++++++++++ +÷++++*++*÷ +++++ ++++÷+++++÷++++++++ + ++++++÷+++' ++

42 5} ...... ! , !...... i r- i i L : + /

7 _ ! -] 420: I J J ] :. J__J. I I ' I [ _l_.J__ I I I : I _ t Z ] I I L _ I I t ! I

0 5 _ 0 15 20 25 30 Days, from 2000-243T00:04 16 42 to 2000-2773-23:551070

Figure 2-11 Period 2000-243 to 2000-277 Cal 1 AGC

gfo.wfc00243-00277 : CAL2 23.0 I I I I : I I _ t , I l I I , + t I t . I _ _ I . 1 I I I I I t i : !

i i

: i r- i i

225 ...... !......

t- +i ...... +K

,,+Ii ...... !...... +...... ,...... i : i

: i

_-,_'+ + + :÷+++ + +4-+ +÷+ + ++ : +_-+++++ +÷ + ++++++++++÷++++ : . +÷+++ + ++ ++ +'÷ : 4.++++++ +

I_ ! i + + : i ! 21.0_L I I I i I J I I i I I J-_J+ : I I I I i I I I I i I I _ • _L- 0 5 10 15 20 25 30 Days, from 2000-243TC.0:C'4:1642 to 2000+277T23:55:1070

Figure 2-12 Period 2000-243 to 2000-277 Cal 2 AGC

December 2000 Page 2-9 From Launch to Acceptance = GFO Altimeter Engineering Assessment Report On-Orbit Instrument Performance -,-

2.2 GFO Cycle (17.05 days) Summaries o

Another indication of the GFO altimeter's internal consistency is the agreement of

m cycle-to-cycle means for: global significant waveheights, sigma-naughts, and wind- M speed. For this analysis, the measurements for complete cycles (17.05 days) were meaned, standard deviations were computed, and measurement histograms were produced. J

Prior to the computations, the data sets were edited to eliminate suspect measurements. Our edit criteria, using Quality Word #1, are as follows:

• Bit 2: Record is zero-filled

• Bit 3: Altimeter not in Fine Track

• Bit 7: No smoothed VATT

• Bit 10: SWH bounds error I • Bit 18: Off-Nadir error

• Bit 19: SWH standard error m J • Bits 22-31: More than 5 frames missing

u • Default fill values indicative of bad data !IB We suggest the use of above criteria by data users for editing the GFO data.

l i The process by which the cycle summaries were produced involved the following cri- m J teria:

• 60 second averaging interval J • 0.2 < SWH < 12.0

• -66.0 < Latitude <66.0 m R • 6.0 < Sigma0 < 16.0

• 45 < Numpoints in intervals < 62 l

m m Ill

December 2000 From Launch to Acceptance Page 2-10 z

w ,,.._ On-Orbit Instrument Performance GFO Altimeter Engineering Assessment Report

All the cycle summaries produced at Wallops so far indicate excellent cycle-to-cycle consistency. Selected cycle summaries are shown in Table 2-1.

Column Definitions for Table 2-1 Cycle Summaries

Day-of-Year YYYY-DDDTHH:MM:SSZ YYYY = Year DDD = Jullian day HH = Hour of the day MM = Minute of the day SS = Second of the day Z = Greenwich Mean Time

Number of Data Points Processed Total number of points processed in the Cycle period used in the Cycle average

Mean SWH Cycle Average Significant Wave Height

= , w Mean AGC Cycle Average Automatic Gain Control

Mean Att Cycle Average Attitude which is calculated from the Off-Nadir Angle

Mean Recvr. Temp. Cycle Average Receiver Temperature

Table 2-1 Cycle Summaries

Number of Mean Mean Mean Mean Start End Data Recvr. SWH Sigma0 Att. Day-of-Year Day-of-Year Points Temp. Processed (m) (db) (deg) (c)

1999-196T00:11:41Z 1999-212T23:59:27Z 715861 2.608 10.773 0.224 38.035

1999-213T00:03:28Z 1999-229T23:34:50Z 58336O 2.501 10.416 0.235 40.290

2000-131T00:01:28Z 2000-147T23:54:52Z 611929 2.676 10.948 0.210 34.753

2000-148T00:32:10Z 2000-164T23:24:42Z 569313 2.494 11.045 0.224 38.570

2000-196T00:03:40Z 2000-212T23:41:53Z 656248 2.519 11.061 0.222 37.971

L 2000-213T00:03:29Z 2000-229T06:20:20Z 367635 2.644 11.I44 0.271 33.709

2000-243T00:01:29Z 2000-259T14:23:31Z 599581 2.485 10.987 0.210 32.913

2000-260T00:01:28Z 2000-277T23:33:42Z 766650 2.594 10.966 0.217 33.070

December 2000 Page 2-11 From Launch to Acceptance GFO Altimeter Engineering Assessment Report On-Orbit Instrument Performance --

2.3 GFO Key Events

The key events for the GFO altimeter since its on-orbit turn-on are summarized in

Table 2-2. These sensor-related key events are extracted from: lib http://gfo.bmpcoe.org/Gfo/Event_Log/gfo_event_log.htm. Additional key events from a Wallops perspectiye have been included in the Table.

Table 2-2 GFO Key Events

Event Date & Time of Event Comments

Launch 10 Feb 1998 1998041TI4:30:00Z Launched from Vandenberg AFB

g Auto Reset 18 Feb 1998 1998049T02:23:00Z Self-initiated Reset (#1A).

Auto Reset 18 Feb 1998 1998049T02:23:00Z Self-initiated Reset (#1B). Again.

U Auto Reset 23 Feb 1998 1998054T11:24:00Z Self-initiated Reset (#2).

Auto Reset 28 Feb 1998 1998059T18:24:00Z Self-initiated Reset (#3.

W Auto Reset 05 Mar 1998 1998064TI9:08:00Z Self-initiated Reset (#4).

Auto Reset 11 Mar 1998 1998070T00:50:00Z Self-initiated Reset (#5). Real time.

Auto Reset 15 Mar 1998 1998074T09:20:00Z Self-initiated Reset (#6).

Manual Reset 18 Mar 1998 1998077T22:25:00Z Manual Reset (#7). J Auto Reset 22 Mar 1998 1998081TI9:08:00Z Self-initiated Reset (#8).

Auto Reset 04 Apr 1998 1998094T08:26:00Z Self-initiated Reset (#9).

Manual Reset 12 Apr 1998 1998102T16:57:00Z Manual Reset (#10).

ManuaI Reset 15 Apr 1998 1998105T23:21:00Z Manual Reset (#11). W Manual Reset 19 Apr 1998 1998109T18:21:00Z Manual Reset (#12).

Auto Reset 29 Apr 1998 1998119T01:22:00Z Self-initiated Reset (#13). m

g Auto Reset 04 May 1998 1998124T03:48:00Z Self-initiated Reset (#14).

Auto Reset 10 May 1998 1998130T15:45:00Z Self-initiated Reset (#15). ! W S/W Change 11 May 1998 1998131T00:00:00Z Algorithm change - Changed delta h init from 796.44002 km to 000.0000 km.

Auto Reset 16 May 1998 1998136T08:01:00Z Self-initiated Reset (#16). !

Manual Reset 21 May 1998 1998141T16:45:00Z Manual Reset (# 17).

m Manual Reset m 22 May 1998 1998142T04:03:00Z ManualReset (#18). II

Auto Reset 28 May 1998 1998148T20:58:00Z Self-initiated Reset (#19). m S/W Change 29 May 1998 1998149T00:00:00Z Algorithm change - Changed delta h init from l W 000.0000 km to -0.019893 km.

'lw

December 2000 m From Launch to Acceptance Page 2-12 l

m On-Orbit Instrument Performance GFO Altimeter Engineering Assessment Report

Table 2-2 GFO Key Events (Continued)

Event Date & Time of Event Comments

S/W Change 09 Jun 1998 1998160T00:00:00Z Algorithm change - Changed swh_lower_hound from 0.00 m to 0.0l m.

S/W Change 09 Jun 1998 1998160T00:00:00Z Algorithm change - Changed swh_upper_bound from 0.00 m to 20.0 m.

SAV Change i0 lun 1998 1998161T00:00:00Z Algorithm change - Changed agc_lower_bound from 0.0 dB to 0.1 dB.

S/W Change 10 Jun 1998 1998161T00:00:00Z Algorithm change - Changed delta agc_init from 32.0 dB to 33.0 dB.

S/W Change 16 Jun 1998 1998167T00:00:00Z Algorithm change - Changed delta h init from - 0.01989 km to 0.020904 kin.

S/W Change 16 Jun 1998 1998167T00:00:00Z Algorithm change - Changed scc.dat file.

Auto Reset 17 Jun 1998 1998168TI7:22:00Z Self-initiated Reset (#20). No data until 08:45Z on 19 Jun (170).

S/W Change 24 Jun 1998 1998175T00:00:00Z Started flight software upload.

S/W Change 25 Jun 1998 1998176T00:00:00Z Algorithm change.

Auto Reset 28 Jun 1998 1998179T11:30:00Z Self-initiated Reset (#21.) No data until 9 July 1998 (190).

S/W Change 30 Jun 1998 1998181T00:00:00Z Algorithm change - Changed delta_swh_init to time_bias and set to -0.049001 sec.

:= -- S/W Change 01 Jul 1998 1998182T00:00:00Z Algorithm change - Changed ra_time delay to 0.0783488.

SN¢ Change 01 Jul 1998 1998182T00:00:00Z To solve sqrt error.

Manual Reset 02 Jul 1998 1998183T20:01:00Z Manual Reset (#22).

Auto Reset 06 Jul 1998 1998187T21:56:00Z Self-initiated Reset (#23).

ManuaI Reset 09 Jul 1998 1998190T21:24:00Z Manual Reset (#24).

ManuaI Reset 12 Jul 1998 1998193T21:32:00Z Manual Reset (#25).

Manual Reset 15 Jul 1998 1998196T21:39:00Z Manual Reset (#26).

Manual Reset 18 Jul 1998 1998199T21:45:00Z Manual Reset (#27).

ManuaI Reset 21 Jul 1998 1998202T21:53:00Z Manual Reset (#28).

Manual Reset 22 Jul 1998 1998203T21:22:00Z Manual Reset (#29).

Manual Reset 24 Jul 1998 1998205T21:59:00Z Manual Reset (#30).

Manual Reset 26 Jul 1998 1998207T20:59:00Z Manual Reset (#31) Payload off; no data received.

Manual Reset 31 Jul 1998 1998212T21:44:00Z Manual Reset (#32).

Manual Reset 03 Aug 1998 1998215T21:50:00Z Manual Reset (#33). Switch to IAP#1.

-qn.*

December 2000 Page 2-13 From Launch to Acceptance

L F GFO Altimeter Engineering Assessment Report On-Orbit Instrument Performance ,._

Table 2-2 GFO Key Events (Continued)

Event Date & Time of Event Comments

Manual Reset 04 Aug 1998 1998216T21:20:00Z Manual Reset (#34). Switch to IAP #2 with Rev. B m Code.

04 Aug 1998 1998216T00:00:00Z Algorithm correction - Changed delta_h_cg from S/W Change u 1.00000 mm to 292.00000 mm.

S/W Change 04 Aug 1998 1998216T00:00:00Z Algorithm correction - Changed delta h init from 0.020904 km to 0.020815 km. n

S/W Change 05 Aug 1998 1998217T00:00:00Z Algorithm correction - Changed RA constant tables.

Payload On 08 Aug 1998 1998220T00:00:00Z Payloads on.

Auto Reset 16 Aug 1998 1998228TI8:I3:00Z Self-initiated Reset (#35). Payloads off.

m Manual Reset 19 Aug 1998 199823 IT00:04:00Z Manual CPU Reset (#36). Payloads off. I

Payload On 04 Sep 1998 1998247T00:00:00Z Payloads on. rl

Auto Reset 10 Sep 1998 1998253T12:20:00Z Self-initiated Reset (#37). m

Auto Reset 15 Sep 1998 1998258TI3:46:00Z Self-initiated Reset (#38).

Maneuver ERO Maneuver. 16 Sep 1998 1998259T00:00:00Z g

Manual Reset 22 Sep 1998 1998265T00:53:00Z Manual Reset (#39). Switch to IAP#1.

Auto Reset 22 Sep 1998 1998265T02:35:OOZ Self-initiated Reset (#40). I

Auto Reset 23 Sep 1998 1998266T01:36:00Z Self-initiated Reset (#41). AC-2 Fault.

Maneuver 08 Oct 1998 1998281T00:00:00Z ERO Maneuver.

Auto Reset 09 Oct 1998 1998282T12:20:00Z Self-initiated Reset (#42).

Auto Reset 19 Oct 1998 1998292T01:36:00Z Self-initiated Reset (#43). m I

Manual Reset 19 Oct 1998 1998292T16:33:00Z Manual Reset (#44). IAP #2 Swap.

S/W Change 20 Oct I998 1998293T00:00:00Z Battery charging level changed from VT 4.5 to VT l 5.0.

Maneuver 21 Oct 1998 1998294T00:00:00Z ERO Maneuver. U Manual Reset 28 Oct 1998 1998301TI8:35:00Z Manual Reset (#45).

Manual Reset 04 Nov 1998 1998308TI4:56:00Z Manual Reset (#46) Rev D- 1 Flight SW Upload. Z lira Manual Reset 05 Nov 1998 1998309T14:26:00Z Manual Reset (#47) Rev D-I Flight SW Upload.

Manual Reset 06 Nov 1998 1998310TI3:56:00Z Manual Reset (#48) Rev D-1 Flight SW Upload.

m I Manual Reset 07 Nov 1998 1998311T15:03:00Z Manual Reset (#49) Rev D-I Flight SW Upload.

Manual Reset 08 Nov 1998 1998312TI4:33:00Z Manual Reset (#50) Rev D-I Flight SW Upload. E J Manual Reset 09 Nov 1998 1998313TI5:41:00Z Manual Reset (#51) Rev D-I Flight SW Upload.

Manual Reset 10 Nov 1998 1998314TI5:I0:00Z Manual Reset (#52) Rev D-I Flight SW Upload. W

From Launch to Acceptance Page 2-14 December 2000 .._. On-OrbitInstrumentPerformance GFOAltimeterEngineeringAssessmentReport

Table 2-2 GFO Key Events (Continued)

Event Date & Time of Event Comments

Manual Reset 11 Nov 1998 1998315TI4:40:00Z Manual Reset (#53) Rev D-1 Flight SW Upload.

Manual Reset 12 Nov 1998 1998316TI5:48:00Z Manual Reset (#54).

Maneuver 13 Nov 1998 1998317T00:00:00Z ERO Maneuver.

Powered Down 16 Nov 1998 1998320T18:43:00Z Manual Reset (#55). Powered down due to onset of Leonid Meteor Swarm.

In Track 20 Nov 1998 1998320T00:00:00Z RA in Trackl. No damage due to Meteors.

Manual Reset 23 Nov 1998 1998327T15:08:00Z Manual Reset (#56).

Manual Reset 30 Nov 1998 1998334T19:49:00Z Manual Reset (#57).

Manual Reset 01 Dec 1998 1998335T16:00:00Z Manual Reset (#58).

Maneuver 02 Dec 1998 1998336T00:00:00Z ERO Maneuver.

Payload Off 03 Dec 1998 1998337T00:00:00Z Payload off.

Payload On 05 Dec 1998 1998339T00:00:00Z Payload on. RA in standby.

Manual Reset 10 Dec 1998 1998344T16:21:00Z Manual Reset (#59).

Maneuver 12 Dec 1998 1998346T00:00:00Z ERO Maneuver.

Manual Reset 16 Dec 1998 1998350T18:I4:00Z Manual Reset (#60).

Auto Reset 22 Dec 1998 1998356TI0:03:00Z Self-initiated Reset (#61).

Manual Reset 30 Dec 1998 1998364TI9:20:00Z Manual Reset (#62).

Manual Reset 06 Jan 1999 1999006T19:03:00Z Manual Reset (#63).

Maneuver 07 Jan 1999 1999007T00:00:00Z ERO Maneuver.

Manual Reset 13 Jan 1999 1999013T23:43:00Z Manual Reset (#64).

Payload Off 18 Jan 1999 1999018T00:00:00Z Payloads off. Transferred operation to Rev. D soft-

ware.

Manual Reset 19 Jan 1999 1999019T18:57:00Z Manual Reset (#65). Flight Software Initialization.

Maneuver 21 Jan I999 1999021T00:00:00Z ERO Maneuver.

Payload On 21 Jan 1999 1999021T00:00:00Z Payloads on. RA in standby.

In Track 29 Jan 1999 1999029T00:00:00Z RA in Track 1.

Battery Test 01 Feb 1999 1999032T00:00:00Z Battery testing. Data outage 1359Z to 1424Z.

Battery Test 04 Feb 1999 1999035T00:00:00Z Battery testing. Data outage 1728Z to 1803Z.

Battery Test 11 Feb 1999 1999042T00:00:00Z Battery testing. Data outage 1850Z to 1925Z.

Maneuver 19 Feb 1999 1999050T00:00:00Z ERO Maneuver.

December 2000 Page 2-15 From Launch to Acceptance GFOAltimeterEngineeringAssessmentReport On-OrbitInstrumentPerformance -,,-

Table 2-2 GFO Key Events (Continued) J

Event Date & Time of Event Comments

Auto Reset 08 Mar 1999 1999067T11:00:00Z Self-initiated Reset (#66A). Anomaly - Bit stuck on g Zero in IAP#2.

Auto Reset 08 Mar 1999 1999067T00:00:00Z Self-initiated Reset (#66B). Failed to Re-load g Rev.D

Auto Reset 08 Mar 1999 1999067T00:00:00Z Self-initiated Reset (#66C). Successful load Rev.B: EEPROM # 1/IAP# 1. g

Payload On l0 Mar 1999 1999069T00:00:00Z Payloads on. RA in standby.

in Track 12 Mar 1999 1999071T00:00:00Z RA in Track 1.

Manual Reset 12 Mar 1999 1999071T23:47:00Z Manual Reset (#67).

Manual Reset 19 Mar 1999 1999078TI7:14:00Z Manual Reset (#68). I

Payload Off 25 Mar 1999 1999084T00:00:00Z Payloads off.

Manual Reset 25 Mar 1999 1999084TI7:26:00Z Manual Reset (#69). Rev.D-I Initialization g Attempt.

Manual Reset 29 Mar 1999 1999088T16:55:00Z Manual Reset(#70). Rev.D- 1 Initialization Attempt.

Manual Reset 02 Apr 1999 1999092T18:20:00Z Manual Reset (#71).

Manual Reset Manual Reset (#72). Rev. D-I Initialization [] 05 Apr 1999 1999095T16:42:00Z ! Attempt. J

Auto Reset 07 Apr 1999 1999097T04:37:00Z Self-initiated Reset (#73).

Manual Reset 12 Apr 1999 1999102T19:49:00Z Manual Reset (#74). J

Manual Reset 13 Apr 1999 1999103TI9:19:OOZ Manual Reset (#75).

Manual Reset 19 Apr 1999 1999109TI6:10:00Z Manual Reset (#76). J

Manual Reset 22 Apr 1999 1999112TI7:51:00Z Manual Reset (#77).

Manual Reset 23 Apt 1999 1999113T01:57:00Z Manual Reset (#78). il

Manual Reset 30 Apr 1999 1999120TI7:06:00Z Manual Reset (#79).

Manual Reset 03 May 1999 1999123T18:50:00Z Manual Reset (#80A). RAM & EDAC Chip Test. g

Manual Reset 03 May 1999 1999123T20:35:00Z Manual Reset (#80B). RAM & EDAC Chip Test.

Manual Reset 03 May 1999 1999123T20:38:00Z Manual Reset (#80C). RAM & EDAC Chip Test. m

Manual Reset 04 May 1999 1999124T16:41:00Z Manual Reset (#81A). RAM & EDAC Chip Vali-

dation. m Uu Manual Reset 04 May 1999 1999124T18:I9:00Z Manual Reset (#81B). RAM & EDAC Chip Vali- dation.

Manual Reset 04 May 1999 1999124T20:I0:00Z Manual Reset (#81C). RAM & EDAC Chip Vali- dation.

m From Launch to Acceptance Page 2-16 December 2000 n w .,_ On-OrbitInstrumentPerformance GFOAltimeterEngineeringAssessmentReport

Table 2-2 GFO Key Events (Continued)

Event Date & Time of Event Comments

Manual Reset 06 May 1999 1999126T17:21:00Z Manual Reset (#82). Swap IAP #2 to IAP #1. Note Daily resets with the upload of software to the IAP. Rev D (1) software.

Manual Reset 11 May 1999 1999131T18:07:00Z Manual Reset (#83).

Manual Reset 12 May 1999 1999132TI7:37:00Z Manual Reset (#84).

Manual Reset 13 May 1999 1999133T18:44:00Z Manual Reset (#85).

Manual Reset 14 May 1999 1999134T18:I4:00Z Manual Reset (#86).

Manual Reset 15 May 1999 1999135T17:44:00Z Manual Reset (#87).

Manual Reset 16 May 1999 1999137T18:52:00Z Manual Reset (#88).

Manual Reset 17 May 1999 1999137TI8:22:00Z Manual Reset (#89).

Manual Reset 19 May 1999 1999139T03:31:00Z Manual Reset (#90).

Manual Reset 20 May 1999 1999140TIS:29:00Z Manual Reset (#91).

Maneuver 20 May 1999 1999140T00:00:00Z ERO Maneuver,

RA On 20 May 1999 1999140T00:00:00Z RA on.

WVR On 22 May 1999 1999142T00:00:00Z WVR on.

Manual Reset 24 May 1999 1999144T21:25:00Z Manual Reset (#92A). Rev D (i) on IAPI & EEPROM #2.

Manual Reset 24 May 1999 1999144T21:25:00Z Manual Reset (#92B). Rev D (1) on IAP1 & EEPROM #2.

Manual Reset 26 May 1999 1999146TI8:41:00Z Manual Reset (#93). IAP1 to IAP2 SWP.

Manual Reset 08 Jun 1999 1999159T20:16:00Z Manual Reset (#94). Swap to IAP2, transmitter #2, and Rev. B.

L_ In Track 09 Jun 1999 1999160T00:00:00Z RA in Track 1. Swap to EEPROM #2 of IAP #1 (Rev D- 1).

Manual Reset 10 Jun 1999 1999161T20:00:00Z Manual Reset (#95).

Maneuver 12 Jun 1999 1999163T00:00:00Z ERO Maneuver.

In Track 15 Jun 1999 1999166T20:00:00Z RA in Track 1.

Cal/Val 15 Jun 1999 1999166T00:00:00Z Began Cal/Val.

Maneuver 24 Jun 1999 1999175T00:00:00Z ERO Maneuver.

Auto Reset 25 Jun 1999 1999176T04:48:00Z Self-initiated Reset (#96).

RA On 28 Jun 1999 1999179T00:00:00Z RA On.

WVR On 28 Jun 1999 1999179T00:00:00Z WVR On.

December 2000 Page 2-17 From Launch to Acceptance GFO Altimeter Engineering Assessment Report On-Orbit Instrument Performance w

Table 2-2 GFO Key Events (Continued) m W

Event Date & Time of Event Comments

Cal/Val 30 Jun 1999 1999181T00:00:00Z Start of first attempt at Cal/Val. g

Data Lost 08 Jul 1999 1999179T00:00:00Z Lost data from 0124Z to 0139Z.

Auto Reset 10 Jul 1999 1999191T04:48:00Z Self-initiated Reset (#97).

Manual Reset 14 Jul 1999 1999195T00:00:00Z Manual Reset (#98). lAP #1 & EEPROM #2 with new BootLoader/VTCW patch. l Manual Reset 21 Jul 1999 1999202T23:05:00Z Manual Reset (#99).

Manual Reset 28 Jul 1999 1999209T02:42:00Z Manual Reset (#100).

Manual Reset 04 Aug 1999 1999216T13:38:00Z Manual Reset (#101).

Maneuver 10 Aug 1999 1999222T01:20:00Z ERO Maneuver. I Payload 18 Aug 1999 1999230T03:30:00Z Payload in standby.

m H/W Failure 26 Aug 1999 1999238T00:00:00Z GPSR #4 software upload completed. Receiver locks onto 8 GPS satellites after auto-reboot. J

Loses Lock 02 Sep 1999 1999245T02:23:00Z GPSR #4 loses lock. NOTE: Since 2 Sep. the GPSR locks after being power cycled, but then fails to retain lock for more than 48 hours.

H Manual Reset 02 Sep 1999 1999245TI5:I8:00Z Manual Reset (#102). IAP#1 - IAP#2, Patched for Failed Bit (Stuck on Zero). g

Manual Reset 03 Sep 1999 1999246T01:53:00Z Manual Reset (#103) EEPROM #2 w/patch, could not go to point per schedule.

Auto Reset 24 Sep 1999 1999267T13:30:00Z Spontaneous Reset (#104). PCU Fault.

Maneuver 30 Sep 1999 1999273TI5:I5:00Z Orbit Adjustment Maneuver. i Auto Reset 05 Oct 1999 1999278TI7:44:00Z Self-initiated Reset (#105).

Commanded 06 OCt 1999 1999279TI4:27:00Z Satellite Commanded to Point. g Manual Reset 14 Oct 1999 1999287T18:38:00Z Manual Reset (#106). VTCW patch uploaded prior to reset.

Maneuver 16 Oct 1999 1999289T00:30:00Z Propulsion Maneuver. ERO maintenance. !1

Auto Reset 16 Oct 1999 1999289T23:25:00Z Self-initiated Reset (#107). Went to acquire sun mode. j Commanded 17 Oct 1999 1999290TI5:33:00Z Satellite Commanded to Point.

Manual Reset 21 Oct 1999 1999294T18:23:00Z Manual Reset (#108). Prior to Patch Upload. ! ul In Point 21 Oct 1999 1999294T18:29:00Z Satellite back in Point.

B i Manual Reset 22 Oct 1999 1999295TI9:28:00Z Manual Reset (#109). II In Point "_'_Oct 1999 1999,.95TI9:35:00Z Satellite back in Point.

l I II

From Launch to Acceptance Page 2-18 December 2000 On-Orbit Instrument Performance GFO Altimeter Engineering Assessment Report

Table 2-2 GFO Key Events (Continued)

Event Date & Time of Event Comments

Maneuver 02 Nov 1999 1999306T00:54:00Z ERO Maintenance Maneuver. 29.76 mm/s with a 0 deg yaw.

Maneuver 16 Nov 1999 1999320T02:01:00Z ERO Maintenance Maneuver. 35.43 mm/sec.

i Manual Reset 17 Nov 1999 1999321T16:06:00Z Manual Reset (#110). Switch to IAP #1, EEPROM #2.

Manual Reset 18 Nov 1999 1999322T18:52:00Z Manual Reset (#111A). Part of IAP Update (left in sun orientation for next reset).

Manual Reset 18 Nov 1999 1999322T20:34:00Z ManuaI Reset (#111B). Second Reset in series, back to point at 2039Z.

Manual Reset 19 Nov 1999 1999323T18:23:00Z Manual Reset (#112). Reset to establish IAP #1, Image 2, with Rev Dvl. To point at 18:28Z.

Maneuver 30 Nov 1999 1999334T03:I0:00Z ERO Maintenance Maneuver. 36.03 mm/s with a 0 deg yaw.

Maneuver 10 Dec 1999 1999344T05:20:00Z ERO Trim Maneuver. 6 mm/s at 180 deg yaw.

Intrusion 17 Dec 1999 1999351T13:46:00Z Moon Intrusion. Caused nadir attitude error of 0.4 degrees for 1 minute.

Maneuver 20 Dec 1999 1999354T22;10;00Z ERO Maintenance Maneuver. 33 mm/s at 0 deg yaw.

Auto Reset 25 Dec 1999 1999359TI8:58:35Z Self-initiated reset - "Smile patch" lost. In standby till activated by a Cal, returned to track. Returned to track 25 Dec 1999, 1999359T23:27:40Z.

Maneuver 29 Dec 1999 1999363T07:15:00Z ERO Trim Maneuver. 3 mm/s at 180 deg yaw.

Manual Reset 06 Jan 2000 2000006T14:29:49Z In standby till activated by a Cal, returned to track. No data available between 006t04:21 and 006t 14:29, patch uploaded during this time. Returned to track 06 Jan 2000, 2000006T23:55:23Z.

Maneuver 17 Jan 2000 2000017T14:45:00Z ERO Trim Maneuver. 22 mm/s at 0 deg yaw.

Intrusion 17 Jan 2000 2000017T21:40:00Z Moon Intrusion. Caused nadir attitude error of 0.475 degrees for 10 sec.

Intrusion 18 Jan 2000 2000018T 14:58:00Z Moon Intrusion. Caused nadir attitude error of 0.520 degrees for 10 sec. Note: The three events on 17, 18 Jan that did not cause the satellite to exceed 0.27 deg.

Maneuver 13 Feb 2000 2000044TI3:35:00Z ERO Maintenance Maneuver.

Configure 23 Feb 2000 2000054T17:29:00Z Reconfiguration. Both Horizon Scanners Selected.

Maneuver 07 Mar 2000 2000067T01:35:00Z ERO Maintenance Maneuver. 27 mm/sec at 0 deg yaw.

December 2000 Page 2-19 From Launch to Acceptance GFOAltimeterEngineeringAssessmentReport On-OrbitInstrumentPerformance ,_

Table 2-2 GFO Key Events (Continued)

Event Date & Time of Event Comments

Maneuver 21 Mar 2000 2000081TI2:45:00Z ERO Maintenance Maneuver. 36 mm/sec at 0 deg l yaw.

Maneuver 01 Apr 2000 2000092T00:18:00Z ERO Maintenance Maneuver. 36 mm/sec at 0 deg yaw.

Maneuver 08 Apr 2000 2000099T00:01:00Z ERO Maintenance Maneuver. 4 mm/sec at 0 deg

yaw. I

Maneuver 08 Apr 2000 2000099T01:41:00Z ERO Maintenance Maneuver. 20 mm/sec at 0 deg yaw.

Maneuver 19 Apr 2000 2000110T0 I:01:00Z ERO Maintenance Maneuver.

Maneuver 19 Apr 2000 2000110T02:41:00Z ERO Maintenance Maneuver. Net result of two maneuvers on 19 Apr, is 42 mm/sec at 0 deg yaw. I

Reconfigure 01 May 2000 2000122T20:40:00Z Reconfiguration. Switch IAPs, Satellite out of point.

Reconfigure 01 May 2000 2000122T22:30:00Z Reconfiguration. Satellite back in point. g

ERO 04 May 2000 2000125T04:00:00Z Satellite back in ERO.

Maneuver 11 May 2000 2000132T 16:25:00Z ERO Maintenance Maneuver. 35 mm/sec at 0 deg yaw.

Maneuver 24 May 2000 2000145T04:48:00Z ERO Maintenance Maneuver. 25 mm/sec at 0 deg yaw. m

Maneuver 16 Jun 2000 2000168T06:15:00Z ERO Maintenance Maneuver. 18 mm/sec at 0 deg yaw. I! Auto Reset 17 Jun 2000 2000169T03:I0:53Z Self-initiated reset - patch lost. In standby till activated by a Cal, returned to track. Returned to track m_ 17 Jun 2000, 2000169TI 1:12:13Z. W

Auto Reset 17 Jun 2000 2000169T 18:59:31Z Self-initiated reset - patch lost. Returned to track 20 Jun 2000, 2000172T00:00:00Z. i Trim Burn 23 Jun 2000 2000175T06:23:00Z Reduce Eccentricity and Argument of Perigee.

135mm/sec at 180 deg yaw. m

Trim Bum 23 Jun 2000 2000175T07:28:00Z Reduce Eccentricity and Argument of Perigee. J 135mm/sec at 0 deg yaw.

L_ Trim Burn 24 Jun 2000 2000176T00:00:00Z Replay of above. Second burn in tile series com- U pleted.

Trim Burn 28 Jun 2000 2000180T02:29:00Z ERO Trim Burn. 24 mm/sec at 180 deg yaw.

u II Trim Burn 20 Jul 2000 2000202TI2:54:00Z ERO Trim Burn. 24 mm/sec at 0 deg yaw.

Trim Bum 04 Aug 2000 2000217T05:00:00Z ERO Trim Burn and Raise SMA. 15 mm/sec at 0 deg yaw.

From Launch to Acceptance Page 2-20 December 2000 On-OrbitInstrumentPerformance GFOAltimeterEngineeringAssessmentReport

Table 2-2 GFO Key Events (Continued)

Event Date & Time of Event Comments

Auto Reset 10 Aug 2000 2000223T11:35:42Z Self-initiated reset - patch lost. Time of reset is approximate due to no data available between 223t06:45 and 223t 11:35. In standby till activated by a Cal to return to track. Returned to track 10 Aug 2000, 2000223TI4:57:46Z.

Auto Reset 10 Aug 2000 2000223T22:52:57Z Second reset - still no patch. In standby till activated by a Cal, returned to track. Returned to track 10 Aug 2000 2000224T02:2 l:30Z.

Manual Reset 11 Aug 2000 2000224T15:47:00Z Commanded power cycle of the RA and upload of patch. Patch still not installed. Time of reset is approximate due to no data available between 224tl 1:51 and 224t15:47.

Auto Reset 12 Aug 2000 2000225T09:00:36Z Self-initiated reset - no patch. In standby till activated by a Cal. Returned to track 12 Aug 2000, 2000225T15:36:07Z.

Manual Reset 14 Aug 2000 2000227TI5:44:29Z In standby. Power cycled and patch uploaded. Returned to track 14 Aug 2000, 2000227T 17:30:16Z.

Auto Reset 16 Aug 2000 2000229T01:04:10Z In Standby till activated by a Cal. Returned to track 16 Aug 2000, 2000229T03:06:I5Z.

Auto Reset 20 Aug 2000 2000233T04:45:51Z Spontaneous reset. In standby. Returned to track without a CAL 20 Aug 2000, 2000233T06:37:23Z.

T Manual Reset 20 Aug 2000 2000233T08:52:00Z RA power cycle to get RA back in track and s/w patch reloaded. Data variables have constant values up to day 2000235t 18:20.

RA Turned Off 22 Aug 2000 2000235TI8:20:00Z Current indications from telemetry are that the receiver power monitor flag is indicating failed and the temperatures are higher than expected.

RA Turned On 23 Aug 2000 2000,.36T03:22:36Z RA and WVR turned on, patch loaded and RA put in track.

Trim Burn 24 Aug 2000 2000237T04:33:00Z ERO Trim Burn. 22.8 mm/sec at 0 deg yaw.

Trim Burn 14 Sep 2000 2000258T18:45:00Z ERO Trim Burn. 28.7 mm/sec at 0 deg yaw.

Trim Burn 29 Sep 2000 2000273T00:53:00Z ERO Trim Burn to Raise SMA. 31.8 mm/sec at 0 deg yaw.

Trim Burn 09 Oct 2000 2000283TI2:28:00Z ERO Trim Burn. 36 mm/sec at 0 deg yaw.

Auto Reset 20 Oct 2000 2000294T23:30:09Z Self-initiated reset - Loss of patch. In standby till activated by a Cal. Returned to track 21 Oct 2000, 2000295T09:00:1 IZ. Patch was not restored.

Manual Reset 21 Oct 2000 2000295T11:11:29Z Power cycled and patch uploaded. Returned to track 21 Oct 2000, 2000295T11:17:18Z.

L..

December 2000 Page 2-21 From Launch to Acceptance ! GFOAltimeterEngineeringAssessmentReport On-OrbitInstrumentPerformance w

Table 2-2 GFO Key Events (Continued) I

Event Date & Time of Event Comments

Trim Burn 26 Oct 2000 2000300T03:40:00Z ERO Trim Burn. 39.2 mm/sec ar0 deg yaw. M

Trim Burn 08 Nov 2000 2000313T03:43:00Z ERO Trim Burn. 36 mm/sec at 0 deg yaw.

Trim Burn 22 Nov 2000 2000327T04:47:00Z ERO Trim Burn. 17.4 mm/sec at 0 deg yaw. Purpose tll is to raise the SMA and maintain the ERO.

Acceptance 29 Nov 2000 2000334T00:00:00Z GFO Acceptance. SPAWAR authorizes DD250s. Q

m I

I I

u = i +

Z I

From Launch to Acceptance Page 2-22 December 2000 i + = Section 3 Assessment of Instrument Performance

In addition to assessing the altimeter's internal measurement consistency, Wallops has also been comparing the GFO results with other spaceborne ocean sensors, and has been examining the altimeter's waveforms.

The following sections report several assessments performed by the WFF GFO team. In some cases, the same parameter was analyzed with different methods and there are some variations in the exact performance estimates that have not been fully explained; however, the results seem to be consistent with each other. All analysis indicates the altimeter instrument is performing within specification.

Section 3.1 addresses the range noise performance. Sections 3.2, 3.4, and 3.5 address the sigma naught parameter and its related wind speed estimates using two different methods. Sections 3.3 and 3.5 address SWH. Section 3.6 describes the SWH and range results of GFO waveform fitting. Sections 3.7 and 3.8 discuss GFO range and SWH estimation consequences of a "smile patch" and of thermal changes. Section 3.9 describes the effects of "sigma0 blooms" in the GFO altimeter data. Early in our GFO altimeter assessment, we studied the acquisition time to provide good data from non- normal tracking and found that the 5 second specification was being met and that GFO over land was tracking and acquiring surfaces better then GEOSAT or TOPEX. We plan to report more details on this in a later report.

3.1 Range Measurement Noise

On behalf of Wallops, Mavis Driscoll and Richard Sailor (both of TASC) have determined the range measurement noise for the GFO altimeter. Driscoll and Sailor (per- sonal communication) employed the same noise determination techniques as they had for all other satellite altimeter mission data.

Their technique is based on observing the noisy time series obtained by differencing repeat tracks to cancel out the geoid. Spectral estimation is then applied to the difference time series, showing 'colored' noise due to oceanography plus a 'white noise floor' due to the sought-after altimeter instrument noise. The rms white noise level is obtained by integrating, from -0.5 to +0.5 Hz, the noise under the noise floor.

Data from 17 pairs of open-ocean repeat GFO groundtracks over a variety of sea states and a variety of geoid signatures were used in the study. The duration of each pair of repeating groundtracks was about eight minutes. Data were edited to remove a few spikes per arc. The resultant estimates of spectra compare very favorably with other satellite altimeter results, as depicted in Figure 3-1 "Noise in Altimetry - Spectra Derived by Differencing Repeat Tracks".

The GFO noise estimates, plotted versus SWH, are shown in Figure 3-2 "GFO Noise Level". For a typical SWH of 2.0 m, the altimeter noise level is observed to be about 2.6 cm.

December 2000 Page 3-1 From Launch to Acceptance GFO Altimeter Engineering Assessment Report Assessment of Instrument Performance ,,,,,

w 10!

mob

We I

= 10.1 W

O 10 "= I

GEOSAT GEOSAT :

IB 10 -2 I i • | • i i _ | : | i I i | , LL. , | | • g i I I # W-4 lO-I 1o -1 lO.I Spatial Frequency (cycles/km)

Figure 3-1 Noise in Altimetry - Spectra Derived by Differencing Repeat Tracks

GFO Noise Estimates from Repeat Tracks

_J |

5- • J __ I

45L " I = =

? o E _ -I 6 i • _ •_ 35 T- 1 i I I 0, m 4, I w O 25_ Z = / t ! i 1 i _.,Fk Lln_ II $- • Ma_,-,,_-_21:O0 "{

i__ C1,5 D - 05 1 15 2 25 3 35 4 ,15 $ IB

Average Significant Wave Height (meters)

Figure 3-2 GFO Noise Level

From Launch to Acceptance Page 3-2 December 2000 Iom Assessment of Instrument Performance GFO Altimeter Engineering Assessment Report

3.2 GFO Sigma Naught Comparison withTOPEX

One of the quantities estimated by a spaceborne radar altimeter is the ocean surface's radar backscattering cross-section. For typographical convenience, this quantity is often referred to as sigma0, sigma naught, or sigma-naught. We will try to use sigma0 throughout this report.

Wallops has compared GFO Sigma0 with TOPEX Ku-Band Sigma0 over concurrent time periods. This was accomplished by averaging global GFO sigma0 over 10-day periods, corresponding to TOPEX 10-day cycles. Since GFO is in a 17 day repeat cycle and TOPEX a 10 day repeat cycle, an approximate 10 day set of GFO data was made using the same start and end time boundaries as the TOPEX cycle.

The TOPEX data, prior to the cycle averaging, were edited, using 60 second averages, to remove any data with Sigma0 > 16.0 dB and Off-nadir > 0.12 deg.

The GFO data, prior to the cycle averaging, were edited, using 60 second averages, to remove any data with Sigma0 > 16.0dB and SWH > 12.0m, and were truncated to delete measurements beyond +/- 66 degrees latitude so as to match TOPEX's latitude coverage.

The results of the Sigma0 comparisons, for 15 TOPEX cycles, are listed ill Table 3-1 "Comparison of GFO and TOPEX (Ku) Sigma0 Means for 10-Day TOPEX Cycles".

Table 3-1 Comparison of GFO andTOPEX (Ku) Sigma0 Means for 10-Day TOPEX Cycles

GFO TOPEX TOPEX Delta GFO TOPEX Number Number Ku Sigma0 Start End Mean Cycle of Data of Data Mean (dB) Day-of-Year Day-of-Year Sigma0 Number Points Points Sigma0 GFO- (dB) Processed Processed (dB) TOPEX

252 1999-198t23:28 1999-208t21:25 413683 331139 10.77 11.11 -0.34

253 1999-208t21:26 1999-218t19:24 358801 333620 10.96 11.20 -0.24

254 1999-218t19:25 1999-228t17:22 392855 331624 10.24 11.14 -0.90

281 2000-121t12:45 2000-13 lt10:42 347018 369181 10.87 11.02 -0.15

282 2000-13 It 10:43 2000-141t08:40 367795 360068 10.85 10.98 -0.13

283 2000-141t08:42 2000-151t06:43 344198 349607 11.07 11.14 -0.07

284 2000-15 lt06:44 2000-161t04:40 300143 349022 11.06 11.09 -0.03

285 2000-16 lt04:41 2000-171102:45 284381 349171 10.96 11.03 -0.07

= 286 2000-171t02:46 2000-181t00:35 142788 342918 10.99 I1.09 -0.10

287 2000-18 lt00:36 2000-190t22:40 384752 342273 10.98 I1.10 -0.12

293 2000-240t 12:27 2000-250t10:25 369654 328836 11.03 11.11 -0.08

294 2000-250t10:26 2000-260t08:23 384319 328926 10.94 11.07 -0.13

December 2000 Page 3-3 From Launch to Acceptance GFOAltimeterEngineeringAssessmentReport Assessmentof InstrumentPerformance ,...,

Table 3-1 Comparison of GFO and TOPEX (Ku) Sigma0 Means Ill for 10-Day TOPEX Cycles (Continued'

GFO TOPEX TOPEX Delta GFO TOPEX Number Number Ku Start End Mean Sigma0 Cycle of Data of Data Mean (dB) Number Day-of-Year Day-of-Year Points Points SigmaO SigmaO GFO- (dB) Processed Processed (dB) TOPEX If

295 2000-260t08:24 2000-270t06:22 415464 315313 10.99 11.13 -0.14

296 2000-270t06:23 2000-280t04:20 431374 326753 10.94 11.08 -0.14

297 2000-280t04:21 2000'290t02:20 412300 334559 10.90 ! 1.06 -0.16

The Table headings are, left-to-right: the TOPEX cycle number, the start date/time and the end date/time for the cycle, the number of GFO data points averaged, the number of TOPEX data points averaged, the GFO mean sigma0 in dB, the TOPEX I mean sigma0 in dB, and the sigma0 difference (GFO minus TOPEX) in dB.

While the sigma0 differences are generally small, that was not the case for the first _mJ__ three listed TOPEX cycles (cycles 252-254) in Table 3-1. At the time, in late-1999, when J these rather large differences were initially noted, Wallops sought the reason. We determined that the sign of the temperature correction to AGC during GFO routine u processing needed to be reversed, and that the temperature correction coefficient needed to be modified. We notified the GFO project of our findings, and they insti- tuted the changes (see Section 4.1). I The 12 subsequent sigma0 differences in Table 3-1 are indicative of the GFO sigma0 improvement resulting from the change in the temperature correction algorithm. The combined cycle 281-287 grouping and the cycle 293-297 grouping have a mean differ- w ence with respect to TOPEX of -0.11dB. Figure 3-3 "Comparison of GFO and TOPEX Sigma0" provides a plot, for the ten-day periods, of GFO sigma0 versus TOPEX sigma0. J

From Launch to Acceptance Page 3-4 December 2000 g Assessment of Instrument Performance GFO Altimeter Engineering Assessment Report

Comparison of GFO and TOPEX for 10-Day TOPEX Cycles 12.0 _-- I I t I ; I I I l '_ I I I I _ I I I I r I i

11.5

¢'n

i ! +, i _11.0 i o i *i _ ! I.J_ (.9

10.5

10.0 10.0 10.5 11.0 11.5 12.0 TOPEX Sigma0, dB -66.0 deg < Latitude > +66.0 deg

Figure 3-3 Comparison of GFO and TOPEX SigmaO

December 2000 Page 3-5 From Launch to Acceptance GFOAltimeterEngineeringAssessmentReport Assessmentof InstrumentPerformance -.,

3.3 GFO SWH Comparison of GFO and TOPEX I

Using the same 15 10-day cycles as for the sigma0 comparison, Wallops has compared the GFO significant waveheight (SWH) with TOPEX Ku-Band SWTI. The SWH J comparison results are tabulated in Table 3-2 "Comparison of GFO and TOPEX (Ku) SWH Means for 10-Day TOPEX Cycles". The Delta SWH (GFO minus TOPEX) in meters is listed in the rightmost column in the table. i

Table 3-2 Comparison of GFO andTOPEX (Ku) SWH Means for 10-Dm,TOPEX Cycles

m GFO TOPEX TOPEX I GFO Delta TOPEX Number Number Ku Start End Mean Cycle of Data of Data Mean SWH(m) Day-of-Year Day-of-Year SWH GFO- Number Points Points SWH m (m) TOPEX Processed Processed (m)

== 252 1999-198t23:28 1999-208t21:25 413683 331139 2.59 2.91 -0.32 ! 253 1999-208t21:26 1999-218t 19:24 358801 333620 2.53 2.81 -0.28

254 1999-218t19:25 1999-228t 17:22 392855 331624 2.54 2.86 -0.32 If 281 2000-121t12:45 2000-13 It 10:42 347018 369181 2.62 3.00 -0.38

282 2000-131t10:43 2000-141 t08:40 367795 360068 2.80 3.17 -0.37 m 283 2000-14 lt08:42 2000-151 t06:43 344198 349607 2.49 2.78 -0.29

284 2000-151 t06:44 2000-161t04:40 300143 349022 2.52 2.88 -0.36 u 285 2000-161t04:41 2000-171t02:45 284381 349171 2.52 2.87 -0.35

286 2000-171t02:46 2000-181t00:35 142788 342918 2.61 2.88 -0.27

287 2000-18 lt00:36 2000-190t22:40 384752 342273 2.51 2.79 -0.28

293 2000-240t 12:27 2000-250t10:25 369654 328836 2.45 2.76 -0.31 ii 294 2000-250t 10:26 2000-260t08:23 384319 328926 2.52 2.81 -0.29

295 2000-260t08:24 2000-270t06:22 415464 315313 2.55 2.85 -0.30 I 296 2000-270t06:23 2000-280t04:20 431374 326753 2.63 2.94 -0.31

297 2000-280t04:21 2000-290t02:20 412300 334559 2.56 2.82 -0.26 m i

; M

=s D

Page 3-6 December 2000 From Launch to Acceptance m i Assessment of Instrument Performance GFO Altimeter Engineering Assessment Report

With respect to TOPEX, the mean GFO SWH delta is -0.31m. The consistency of the delta for a variety of seastates is depicted in Figure 3-4 "Comparison of GFO and TOPEX SWH".

4.°i ...... S

_ 3.0 ...... I J :.:'i ! 25]- ...... _ ...... !

2.0 = , _ , _: , , , __._L___ , _ _ i _ , 2.0 2.5 3.0 3.5 4.0 TOPEX SWH, m -66.0 deg < Latitude > +66.0 deg

Figure 3-4 Comparison of GFO and TOPEX SWH

3.4 Wind Speeds

A wind speed comparison study has been carried out between the GFO altimeter and the SeaWinds scatterometer on board the NASA QuikSCAT satellite (QSCAT). For each GFO measurement, the closest scatterometer footprint is selected whenever it is within 25 km and within 60 min of the GFO location. Due to various orbit character- istics, geographical distributions of data are very different from one sensor to the other. The phasing is such that all the collocated data are at high latitudes, above 50 degrees, during the period of 17 days from 3 May 2000 to 19 May 2000 (Figure 3-5). Figure 3-6 presents the comparison between the GFO and QSCAT wind speeds. The GFO wind speeds are generally larger than the QSCAT ones. The bin-averaged data in Figure 3-7, computed by filtering the outliers, show a difference between the two sources of about 2 m/s in the wind speed range from 5 to 12 m/s.

Due to the limitations of the number of crossovers between GFO and QSCAT, and their locations at latitudes above 50 degrees which cannot give a good representation of the wind speed distribution over the entire globe, we additionally chose to collo- cate GFO data with interpolated surface model winds from the National Centers for Environmental Prediction (NCEP). By then reproducing the same procedure with the TOPEX altimeter data, we are able to compare the two altimeters using the same meteorological wind speed reference. Our comparison study leading to GFO altimeter sigma0 and significant wave height calibration correction with respect to TOPEX

December 2000 Page 3-7 From Launch to Acceptance GFO Altimeter Engineering Assessment Report Assessment of Instrument Performance i.,

measurements is enclosed in this document as Appendix A, and is further addressed in Section 3.5.

Start date: 124, End date: 140, Numberof data: 1980 90 lij

80 7oI 60' J 4o

_ 20 L=-- _._ lo Wl

"_100

_ 2o I 30 40

50 60 70

80 90 0 30 60 90 120 150 180 210 240 270 300 330 360 Longitude(deg) I

Figure 3-5 Distribution of Open Ocean Collected Data

Startdate:124,Enddate: t40, Numberof data:1980 30 T i t f _ l J 1 i _ i ,i i .,1/ 28 I

26 UGF O = 0.853 UQSCAT+ 3,05 .FE_. _1_

_'--" 22 R2= 0.88 _f_/ ¢d) E 20 ;..;... • i " , U m14 • ","L:'il'__: ."

•_ 12 •,.',,,.-,i _-_..__-d:_:'i- 010 ii • " • _ 8 ";,II.fl'l l:.lt!_. '" ". • " , ::±JTI'I_ :'" " • 6 4 _ ."i'i_:;_ " . . • 2 :':brl..i.-.. • W 0 _zl" 1 "l .'_" , , I ,I I L I _ , I 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 3O QSCATwind speed(m s i)

Figure 3-6 Comparison Between GFO and QSCAT Wind Speeds

From Launch to Acceptance Page 3-8 December 2000 Assessmentof InstrumentPerformance GFOAltimeterEngineeringAssessmentReport

Start date: 124, End date: 140, Number of data: 1943

2O I t I I I I I I

18 -T:

16 ...... _ ...... _...... _...... i...

_14 E

_12 ...... :...... :...... i- - - T- T

"o r.- -_ a © u_ 6

4 :...... :...... : ......

2 ..... "....

0 L f i i , , i 0 2 4 6 8 10 12 14 16 18 20 QSCAT wind speed (m s 1)

Figure 3-7 Standard Deviation Comparison Between GFO and QSCATWind Speeds

3.5 GFO Sigma0 and SWH Calibration Correction

The study in this report's Appendix A used NCEP as an intermediate reference for comparison of the GFO and TOPEX sigma0 and SWH. This study showed that the GFO cross-section values are biased about 0.37 dB lower than the TOPEX cross-section, and the GFO significant wave height values are biased about 0.24 m lower than the TOPEX significant waveheight. Attachment A ("GFO altimeter sigma0 and SWH calibration correction," by Tran, et al) describes how these GFO calibration corrections were generated, and recommends these additive contributions to both the GFO cross-section and significant waveheight to improve the wind speed retrieval.

The correction to GFO sigma0 leads to statistically significant global error reductions for the GFO windspeed. The overall bias between GFO and NCEP windspeeds decreases from 1.75 m/s to 0.53 m/s, and the root-mean-square (rms) error decreases from 2.44 m/s to 1.59 m/s with the operational algorithm. L

3.5.1 Implementation

The recommendation for the Sigma0 correction was implemented in the Payload Operations Center (POC) Systems effective 7 December 2000. The first ngdr dataset with the "AGC CALIBRATION BIAS = -0.370000" was 2000342_00835_86399.

December 2000 Page 3-9 From Launch to Acceptance GFO Altimeter Engineering Assessment Report Assessment of Instrument Performance .-

3.6 Waveform Fitting in GFO Data

This section will describe results from waveform fitting to 1-second averages of GFO waveform data but first will examine typical GFO Calibration Mode I and Calibra- U tion Mode 2 waveform data. The first thing to do is to verify that the GFO point-target response, in effect the transmitted pulse shape as observed by the altimeter's receiver, has the expected sinc^2 shape, where sinc x = (sin x) / x. The GFO Calibra- U tion Mode 1 samples the point-target response at the fundamental altimeter sample spacing of 3.125 nanoseconds, and Figure 3-8 shows the 2000 day 147 Ca] Mode 1 waveform samples and a least-squares fit of a sinc^2 function to those waveform samples. The vertical axis of the upper plot in Figure 3-8 is in waveform amplitude counts which is proportional to power. Only the first several sidelobes of the actual

Cal Mode I waveform data are plotted in these figures, because the rest of the values U have no meaning as a result of the finite resolution of the 8-bit resolution of the individual GFO waveform samples. The lower plot of Figure 3-8 shows the same data as the upper plot except that the vertical axis is in dB relative to the peak value of the u sinc^2 fit. These results indicate that the actual point-target response is a very good approximation to the theoretical sinc^2 shape.

In the GFO Calibration Mode 2 the waveform samplers look at receiver thermal noise only, with no signal from the transmitter. Ideally the noise spectrum should be flat and the Cal Mode 2 observed amplitude would be the same in each of the GFO altimeter's 128 on-board waveform samples. The upper plot of Figure 3-9 shows the waveform samples from a typical GFO Cal Mode 2 waveform on 2000 day 147, together with the individual sample standard deviation. The lower plot of Figure 3-9 shows J the individual sample waveform gain adjustments derived from the data of the

= = upper plot and from the assumption that upper plot should have been completely E_ flat. Notice that the gain adjustments from Cal Mode 2 are only of the order of a couple of tenths of a percent, extremely Small corrections in a practical sense.

As the Cal Mode 1 and Cal Mode 2 results indicated adequacy of a sinc^2 model for m the point-target response and flatness of the waveform sampler relative response, we assembled a set of GFO over-ocean waveform sample 1-second averages for portions of 2000 days 124, 134, 137, and 150. These data were edited to be over deep water only, in latitudes where no ice could be present, and the average waveforms were then least-squares fitted by a zero-skewness waveform model having five variable fit parameters: amplitude, delta range, SWH, noise baseline, and attitude angle. The fit J results were edited to remove obvious bad fits, and we were left with about 170 minutes of 1-second fit results. Figure 3-10 shows typical fit results for the 1-second

waveform averages: one example is at relatively low SWH and the other example is u at relatively high SWH.

The second of the five fit parameters, the delta range, indicates how far the fitted = = J model waveform is away from the altimeter's track point (the track point is sample 32.5 of the waveform sample set numbered 1 - 128). This waveform fit delta range will be a function of attitude angle and SWH; in fact it should agree with the attitude/SWH range correction in the GFO data processing if the GFO attitude/SWH range correction is correctly implemented. The attitude/SWH range correction is dif-

From Launch to Acceptance Page 3-10 December 2000 I Assessment of Instrument Performance GFO Altimeter Engineering Assessment Report

ferent for the five different gate index values in the GFO tracker. Gate index I occurs for the lowest SWH values and gate index 5 for the highest, but most of the GFO over-ocean operation will use gate index values 2 or 3. Figure 3-11 and Figure 3-12 show the differences between the waveform fit delta range and the GFO attitude/ SWH range correction as a function of the GFO SWH, in separate plots for the four separate gate index values in our trial waveform data set. There were no gate index 5 values in the data. As indicated in the legends on the plots in Figure 3-11 and Figure 3-12, the waveform fit delta range and the GFO attitude/SWH range correction agreed on average to a couple of centimeters or so. The errors in the comparison could presumably be driven down by comparing a lot more data, but we think that Figure 3-11 and Figure 3-12 indicate that the GFO attitude/SWH range correction is performing well within the needs of the GFO mission.

Figure 3-13 compares the waveform fit attitude angle estimates with those from the GFO SDR (or NGDR). The 1-second waveform fit attitude estimates are noisier than the GFO SDR attitudes, because the GFO attitude is computed from the fitted Vatt which is a highly smoothed quantity. However Figure 3-13 indicates that the agreement of waveform fit attitude and GFO SDR attitude is very good, within 0.04 degrees on average. The attitude angle is of no particular interest in itself except in its use in correcting sigma0 estimates, and the agreement shown in Figure 3-13 is also within the needs of the GFO mission.

Finally, Figure 3-14 and Figure 3-15 compare the 1-second waveform fit SWH estimates with those on the GFO SDR (or NGDR); as in Figure 3-11 and Figure 3-12 there are separate subplots for the four gate index values in the waveform data set. There appears to be a consistent small bias between two SWH estimates, with the waveform fit SWH estimates being 0.1 to 0.2 m higher than the GFO SDR SWH estimates. This small SWH difference is very well within the limits set by the GFO requirements and mission needs.

There is one small question about GFO waveform data. The noise baseline values are too good, i.e., too low for the nominal 13 dB GFO signal to noise ratio (SNR). As an example, Figure 3-15 shows a magnified portion of the typical GFO waveforms already shown in Figure 3-10. Notice for the low SWH example that equivalent waveform sample 28 (telemetry sample 20) and below have zero amplitude. For the higher SWH example all equivalent waveform samples 18 (telemetry sample 10) and below have zero amplitude. For a 13 dB SNR and the typical waveform maximum value of 1100 in Figure 3-10, the average waveform value in the early noise-only region should have been around 55, not zero.

This GFO situation is different than our experience with data from the TOPEX radar altimeter. The original TOPEX SNR specification was 13 dB but the Ku-band system actually had about 6 dB of additional margin, for a system SNR of almost 20 dB. The ;,_... typical TOPEX Ku-band waveform has a maximum amplitude of about 8000, and the noise baseline in the early samples is typically between 80 and 100. For the GFO maximum waveform amplitude of 1100 we would expect to see noise baseline values of around 11 if the GFO SNR were as high as 20 dB. It is true that because the waveform samples are telemetered in 8-bit words there are limits on how small a non-zero

December 2000 Page 3-11 From Launch to Acceptance GFO Altimeter Engineering Assessment Report Assessment of Instrument Performance ,.,-

waveform sample will be, but for GFO we would still expect to see non-zero sample values as low as 6 or so, solely based on the telemetry limits. A GFO early noise region signal of 6 would imply a SNR of about 23 dB. That all the early samples are zero therefore implies a highly unlikely SNR of greater than 23 dB for GFO. m

These numbers are approximate, for illustration only, to indicate why we are sur- prised to see zeros for all the early waveform samples in GFO. We had noticed that M the baseline looked too clean in the preflight GFO data, and this same behavior per- sists in GFO on-orbit data. Although only the two examples in Figure 3-17 are presented in this report, we see the zero values in early waveform samples in all GFO J data. This too-clean baseline should have no effect on GFO's performing to its specifications in range and SWH estimation, and we point it out here only as a curiosity.

From Launch to Acceptance Page 3-12 December 2000 I Assessment of Instrument Performance GFO Altimeter Engineering Assessment Report

GFO Calibration Mode 1 Waveform

from 2000 day 147 45,000 • Cal 1 data 40,000 Cal 1 fit

35,000 E 30,000

25,000 c 20,000 E E 15,ooo i O i 10,000

5,000

0 .... --"-_"_'_ ---

-12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12

Distance from track-point, multiples of 3.125 ns

GFO Calibration Mode 1 Waveform

from 2000 day 147

• Cal 1 data ÷ _ -- Cal 1 sinc"2 fit i t ' I

0 2 4 6 8 10 12 Distance from track-point, multiples of 3.125 ns

Figure 3-8 GFO Waveform Data from Calibration Mode 1 Fitted to Ideal Sinc^2 with 3,125 ns Width

December 2000 Page 3-13 From Launch to Acceptance GFO Altimeter Engineering Assessment Report Assessment of Instrument Performance =-

GFO Calibration Mode 2 Waveform from 2000 day 147 493 m

_491

_ 490

m lib

469

488

mi 487 ......

486 i 0 8 16 24 32 40 48 56 64 72 80 88 96 104 112 120 128 Equivalent waveform sample (1-128)

GFO Waveform Multiplicative Factors from Cal 2 from 2000 day 147 1.006 I i • 1.005 _ ...... i 1_o 1.004 ! . i 4 1.0o3_ ,

o 1.002 _ " , = - t 1.001 = "...... =...... lib

E t A _ • •• • • •

1.oool," • • • A • O ] A• A • "_ 0.999 ii " " " " "" " " >_. A • • • ! i ] ,,, • , w 0.998 _ ==" • , • * • ,i i 0.997 ' J 0 8 16 24 32 40 48 56 64 72 80 88 96 104 I12 120 128 Equivalent waveformsample (1-128)

BIB

. _=

Figure 3-9 GFO Waveform Data from Calibration Mode 2 i

m m It

m ill

From Launch to Acceptance Page 3-14 December 2000

i ) Assessment of Instrument Performance GFO Altimeter Engineering Assessment Report

Typical GFO Low SWH Waveform Fit on 2000 Day 137 for 1-second waveform averages 1200

1100

1000

900 i 800

_" 700 i __ ..... -

600 o 500 '_ 400 i • Input at 2000-137T22:27:52,69 300

200 i!l -- fit SWH=1.09 and ATT=0.25 ,,..._. 100 I

8 16 24 32 40 48 56 64 72 80 88 96 104 112 120 128 Equivalent waveform sample (1-128)

Typical GFO High SWH Waveform Fit on 2000 Day 137 for 1-second waveform averages 1200

1100

1000 900 800

_" 700

E 600 500 I .... 400

300 I e 200

100 ' -- " fit SWH=8.45 and ATT=0.13 0 0 8 16 24 32 40 48 56. 64 72 80 88 96 104 112 120 128 Equivalent waveform sample (1-128)

Fig ure 3-10 GFO Waveform Sample Data and Model Waveform Fits

December 2000 Page 3-15 From Launch to Acceptance GFOAltimeterEngineeringAssessmentReport Assessmentof InstrumentPerformance ..

GFO dR(Att/SWH) Difference (Fit - SDR) for Gate Index 1 # 1-sec avg's = 338, Mean =0.93, StDev =1.56, & StDev of Mean =0.08 Bi 25

20 E ¢J 15 W ..¢ n" O t0 o o | o ,(,.., 5 ° i

O ,$ oO o °_ + o _-%oo_oo_,>-_.-o o + o°_o+++oq;o_o • -o °° > m -5 m

L. -10 g O O -15

m -20 [] -25 ...... 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1,0 t+1 1.2 1+3 SDR SWH value, m

GFO dR(Att/SWH) Difference (Fit - SDR) for Gate Index 2 # 1-sec avg's = 4932, Mean =i.42, gtDev --2.17, & StDev of Mean =0.03 25

2o E O BB

t_ u_ 10 o + °o oO +_ ,%° °%° o o _ <> J o ° ,ii+,% o o o o o o <,o o ° o P_.o :_° o '+o + ++/° + U E °° o i+°+ :> m -5 o o IB L.. -10 O I, fJ -15

-2O

-25 1.0 1.5 2+0 2,5 3.0 3.5 m SDR SWH value, m R

j *- Figure 3-il GFO Waveform Fit Comi_arisons of Attitude/SWH Correction to Range for Gate Index 1 and 2

From Launch to Acceptance Page 3-16 December 2000

BII • r Assessment of Instrument Performance GFO Altimeter Engineering Assessment Report

GFO dR(Att/SWH) Difference (Fit - SDR) for Gate Index 3 # 1-see avg's = 4107, Mean =1.12, StDev =3.17, &StDev of Mean =0.05 25

20 E t_ 15 o '_ o e e 10 - . , °°° °: .-Lo';°op°°. oo ° oy.[ ° 5

E 0 o '.T N , o °o -5 #+o* o

o o o o ° o , _ o o iJ o,_ • 4 }.. -10 o tj o -15 0

-20

-25 25 3.0 3.5 40 4.5 50 55 60 65 70 SDR SWH value, m

GFO dR(Att/SWH) Difference (Fit - SDR) for Gate Index 4 # 1-sec avg's = 845, Mean =0.45, StDev =5.26, & StDev of Mean =0.18 25

20 E : ° 15 ,.4 %P ° °

10 +- o° +..<,,+.o_O_°°° * '_o° _°°o #+ °°° ° * ° ° _ o :_+,.+11L:<,"_+_ + • 6" ° °0 o , ++ ._ ° =.,-. + _ +<,_od,+,l_+._,,°+<,#_+,e:.,,+,:+ : ° ° o o° °° + +,_",+_t,P,J1+..,,.."l,+_+_'_tt 0 _+ + ° ° ",, ° ° • _. "_ " U + o° + _ +o+ + o I

41- ° o o -5 o_++,,,.o:"_°"IP °%+ o_ °+o o .+ ° o o o

v_. -I0 '_ °+]°_ _ o:° ° + oo++" , ° ° ° ° 0 ° ° o o

-15 o °

: o -20

° -25 6 7 8 9 10 11 12 13 SDR SWH value, m

Figure 3-12 GFO Waveform Fit Comparisons of Attitude/SWH Correction to Range for Gate Index 3 and 4

December 2000 Page 3-17 From Launch to Acceptance GFOAltimeterEngineeringAssessmentReport AssessmentofInstrumentPerformance ,.-

1-sec data avg's from portions of 2000 days 124, 134, 137, and 150 0.3 straight line fit: intercept = -0.036 and slope = 0.051 i

iD o o ooo

rr_

-0.4 0.05 0.10 0.15 0.20 0.25 0.30 SDR attitude, degrees

Figure 3-13 GFO Attitude Diff. (Fit - SDR) for All Gate Index (1-4) I

m I

all

z U

m m

From Launch to Acceptance Page 3-18 December 2000 _I BI Assessment of Instrument Performance GFO Altimeter Engineering Assessment Report

GFO SWH Difference (Fit - SDR) for Gate Index 1 # 1-sec avg's = 338, Mean =0.17, StDev =0.08, & StDev of Mean =0.004 125

1.00

E 0.75

0.50 (n o_ o o • e^ _°0_ ¢ o _ _qo oo 0 25 __

o 0.00 o o o > -0.25

,,t,- -0.50

"o -0.75

-1.00

-1 25 ....

0.2 0.3 0.4 0.5 0,6 0.7 0.8 0,9 1.0 1,1 1.2 1.3

SDR SWH value, m

GFO SWH Difference (Fit - SDR) for Gate Index 2 # 1-sec avg's = 4932, Mean =0.14, StDev =0.12, & StDev of Mean =0.002 1 25

1.00

E 0.75 .... + _' - -

=t-, 0.50 - .: :'=- o°2 - 0.25 °._. "" - ° " _o -

0.00 _ ,_ _ o o oo

-0.25 ...... ° + o

,,r -0.50 - _ -- : O3 I0

-0.75

-1.00 ...... -i.25

1.0 1.5 2,0 25 3.0 3.5

SDR SWH value, m

Figure 3-14 GFO Waveform Fit Comparisons of SWH Estimates for Gate Index 1 and 2

December 2000 Page 3-19 From Launch to Acceptance GFOAltimeterEngineeringAssessmentReport Assessmentof InstrumentPerformance .,,,

GFO SWH Difference (Fit - SDR) for Gate Index 3 BB # 1-see avg's = 4107, Mean =0.16 StDev =0.19, & StDev of Mean =0.003 1.25 . + + ! ! ! P z

1 l O0 _ • Q I • o

E 0.75 _ + +o* -_ "-- + °°° <,0 " ° <'o ..... °°o _:._,_,o °+.....8 °° Z°°°:._o °eoe . ° ++_ ° 050 eel, _ . _ o ee o z ! I _= o.25 ° ++,,,.

o.oo

i -0.25 B

_ ° _ • o : I> , o o + ° • • -o.5o o O) e -0.75 i

-1.00

-1 25 2.5 3.0 3.5 4.0 45 50 55 60 6.5 7.0 z i SDR SWH value, m

= =

GFO SWH Difference (Fit - SDR) for Gate Index 4 i # 1-sec avg's = 845, Mean =0.22, StDev =0.26, & StDev of Mean =0.009 1.25 o

o o 1+00 ++ e _ o e =__ E o ,? ° o Oo Im 0.75 n- °,.. _%; °I,_+*+_ +++ +" +0__ ..... ° ° 00 0.50 i ,_+,#+o+,#":°°°I+o°°° _o °° ° ° o_ +, + ° ',,

0.25 ; J,;:_,_.-_o_Ir_'-_'IE_.+;_-.+*_ ° } ° = ° °o- ° ± + + ,,. _' ++++ U o o + I o ,=, o o °o • 0.00 -+'_,"ii_$'%Ic_.5,,IL-l,++° °++ '° ° " ° ° ° + o"+.+o_+or+E+"- ' +°'+++ °°°° ° °; ° ° ° -0.25 +e° _ #¢_ ° o + o ° i q- -0.50 - o o o

-0.75 ° 2 -1.00 _

-1 +25 6 7 B 9 10 1I 12 13 SDR SWH value, m IIIB

Figure 3-15 GFO Waveform Fit Comparisons of SWH Estimates for Gate Index 3 and 4

IB +

From Launch to Acceptance Page 3-20 December 2000 i _ Assessment of Instrument Performance GFO Altimeter Engineering Assessment Report

Leading Edge Detail, GFO Low SWH Waveform Fit for 1-second waveform average 9O o input at 2000-137T22:27:52.69 807o i...... --,,, _w.=,o,.n,,__-o:_ _ 6o

_E30 i

m 20

10 " w ] -10 10 15 20 25 30 35 Equivalent waveform sample (1-128)

Leading Edge Detail, GFO High SWH Waveform Fit for 1-second waveform average 9O

80 j ' if:P: tw :t=8.:: 0:: ld37:2:::.31:: 3" 37 / ! 70 ¢D _6o }5o

= _ 40

o30

_ 20

-10 I i 10 15 2O 25 30 35 Equivalent waveform sample (1-128)

w- Figure 3-16 GFO Noise Baseline Examples

December 2000 Page 3-21 From Launch to Acceptance GFOAltimeterEngineeringAssessmentReport AssessmentofInstrumentPerformance ,-,,

z 3.7 GFO"Smile Patch"and its Consequences I

Ideally the Calibration Mode 2 should be flat, meaning it should have uniform output across the set of waveform samples. Relatively late in the GFO preflight testing it m was noted that the Calibration Mode 2 data exhibited a "smile" pattern, with the samples at either end of the set having higher effective gain than the samples in the middle of the set. The flight software had already been developed and installed. Since the on-board algorithms were designed assuming a flat waveform sample response, a software patch was developed by the altimeter developers at E-Systems to be uploaded to the GFO altimeter. This "smile patch" is basically a set of 128 multi- l plicative waveform sample adjustments, gains in effect, with a "frown" shape to compensate for the smile pattern. The patch values are not well documented so we will list them here. Table 3-3 lists the GFO smile patch values for a waveform set numbered 1 through 128.

Table 3-3 GFO "Smile Patch" Values for Waveform Samples I - 128

Waveform Patch Waveform Patch Waveform Patch Waveform Patch Sample # Value Sample # Value Sample # Value Sample # Value i 1 0.953013 33 1.011715 65 0.959713 97 0.945683

2 0.953013 34 1.010947 66 0.959713 98 0.945683 m I 0.956746 35 1.011187 67 0.959713 99 0.945683

0.956746 36 1.012150 68 0.959713 100 0.945683 J 0.959180 37 1.012316 69 0.983433 101 0.934026

6 0.959180 38 1.016388 7O 0.983433 102 0.934026 M 7 0.963749 39 1.016750 71 0.983433 103 0.934026

m 0.963749 40 1.015712 72 0.983433 104 0.934026 m 0.966890 41 1.015808 73 0.978832 105 0.921097

10 0.966890 42 1.016903 74 0.978832 106 0.921097

11 0.970482 43 1.017324 75 0.978832 107 0.921097

44 1.018602 76 0.978832 108 0.921097 12 0.970482 m ,

13 0.976526 45 1_016327 77 0.981061 109 0.909145

14 0.976526 46 1.017834 78 0.98106i 110 0.909145

15 0.980738 47 1.016709 79 0.981061 111 0.909145

16 0.980738 48 1.014588 8O 0.981061 112 0.909145

17 0.979146 49 1.011463 81 0.982289 113 0.896047

18 0.986360 5O 1.011463 82 0.982289 114 0.896047 _B 19 0.987668 51 1.004834 83 0.982289 115 0.896047

From Launch to Acceptance Page 3-22 December 2000

i Assessment of Instrument Performance GFO Altimeter Engineering Assessment Report

Table 3-3 GFO "Smile Patch'Values for Waveform Samples 1 - 128 (Continued

Waveform Patch Waveform Patch Waveform Patch Waveform Patch Sample # Value Sample # Value Sample # Value Sample # Value

20 0.989496 52 1.004834 84 0.982289 116 0.896047

21 0.992460 53 1.000536 85 0.976700 117 0.884578

22 0.995203 54 1.000536 86 0.976700 118 0.884578

23 0.995020 55 0.996737 87 0.97670O 119 0.884578

24 0.996716 56 0.996737 88 0.976700 120 O.884578

25 0.997351 57 0.994990 89 0.967161 121 0.873742

26 0.999910 58 0.994990 90 0.967161 122 0.873742

27 1.004239 59 0.996192 91 O.967161 123 0.873742

28 1.004929 60 0.996192 92 0.967161 124 0.873742

29 1.004551 61 0.997144 93 0.955166 125 0.864794

30 1.006474 62 0.997144 94 0.955166 126 0.864794

31 1.005611 63 0.957526 95 0.955166 127 0.864794

32 1.008372 64 0.957526 96 0.955166 128 0.864794

The data of Table 3-3 are plotted in Figure 3-17.

1.03

olINP°Ol_e 1.01 o6e_°_ _o

0.99 j "."

_0.97

J_ u N o _.o.95 N m E N 0.93

o0.91 m IJL £9 0.89

0.87

0.85 0 8 16 24 32 40 48 56 64 72 80 88 96 104 112 120 128 GFO on-board sample # (1-128)

Figure 3-17 GFO Waveform "Smile Patch'Values

December 2000 Page 3-23 From Launch to Acceptance GFO Altimeter Engineering Assessment Report Assessment of Instrument Performance ...,

Among the GFO events listed in Table 2-2 is a system self-initiated reset on 25 December 1999 in which the smile patch was lost. The GFO altimeter operated without the smile patch until 06 January 2000 when the smile patch was reinstalled. The altimeter algorithms and ground processing corrections for SWH and attitude effects f all assume the patch to have been present, and there will be range and SWH errors in the system output (i.e., in the distributed GFO results) for data taken during the smile patch's absence. We'll refer to these as the "no-patch" errors. mg

Now we will describe a simulation exercise which we carried out to assess the magni- m

tude of the no-patch errors. The simulation assumed 800 kilometers satellite height I and 1.6 degrees antenna beamwidth. We have a digital computer program which simulates the response of the GFO altimeter to a model altimeter waveform, and we executed that program for a large set of SWH values between 0 and 20 meters and attitude values between 0.0 and 0.4 degrees under the assumption of perfectly uniform waveform sample response. From this set of "perfect GFO" simulation data we built a large table of GFO range and SWH corrections as a function of SWH and atti- P tude. Then we ran the altimeter simulator for waveforms which the altimeter's tracker would have seen if the smile patch had been omitted (i.e., with the "frown" pattern present in the waveform sample gains); we produced simulated GFO no- U patch output for a range of SWH and attitude inputs. We used the no patch outputs to enter the correction look-up tables which had been produced for the "perfect GFO". in this way we could estimate the range and SWH errors for GFO no patch u data at the output of the GFO ground processing (and correction) system.

Figure 3-18 shows the simulation's estimate of the no-patch error in GFO range for m

90 m M

8o mAtt = 0.3 • Att = 0.2 E 70 • Att = 0.1 J E

i. e 60

e 50 U o

.,- 40 0

m g 3o C

2O loj I

m 0 m 0 2 4 6 8 10 12 14

SWH, m

! Figure 3-18 GFO'_ange Correcii6n Err6/:VS. _WH, With NO Patch D

SWH from 0 to 14 m for the three attitude values 0.1, 0.2, and 0.3 degrees. This range R I

From Launch to Acceptance Page 3-24 December 2000 R Assessmentof InstrumentPerformance GFOAltimeterEngineeringAssessmentReport

error has relatively little attitude dependence, and varies almost linearly from a 1 centimeter error at very low SWH to a 8 centimeter error at 14 m SWH. Figure 3-19 shows the corresponding simulation estimate of the GFO no-patch error in SWH, for the same SWH and attitude ranges as Figure 3-18. The no-patch SWH error is not as easily characterized as the range error, but the SWH error is in all cases less than 0.15 m for SWH up to 14 m. The obvious breaks or discontinuities in Figure 3-19 are

-0.04 0 2 4 6 8 10 12 14

SWH, m

Figure 3-19 GFO SWH Error vs. SWH, With NO Patch

where the GFO altimeter switches between its five gate index values. The same gate index discontinuities are visible in Figure 3-18 except at the switch from gate index 1 to gate index 2.

F Details of the actual GFO ground correction algorithms could change the range error by a centimeter or two. There are no doubt a number of other small effects not included in our simulation. It was not the intent of this section to produce a correc- w tion to be applied directly to the GFO no-patch estimates, but we have showed the importance of the patch, as well as providing some estimate of the errors in GFO data should the smile patch ever again fail to be uploaded after a system reset.

December 2000 Page 3-25 From Launch to Acceptance GFO Altimeter Engineering Assessment Report Assessment of Instrument Performance -.,,

3.8 GFO Range and SWH Consequences of Thermal Change U

The GFO on-orbit temperature generally is within 5 degree C of a mean temperature

somewhere around 35 C. See for example Figure 2-4 which plots a GFO receiver tem- W perature for parts of year 2000. Later in this report in Section 4.1 there is an analysis of temperature effects on AGC and sigma0. One consequence of a GFO temperature change is a change in the relative gains of the waveform samples. The Calibration m Mode 2 looks at the waveform sample response to a noise-only signal which should be uniform across the sample set, so that all waveform samples would ideally have the same average value in this calibration mode. When the Calibration Mode 2 result m is not completely uniform, the renormalized inverse of the Calibration Mode 2 output can be taken as a set of corrective gains. See Section 3.7 for a discussion of the "smile patch" gains derived from Calibration Mode 2 data. J

The smile patch removes most of the nonuniform response of the waveform samples, but there are expected to be small residual temperature effects. That is, the Calibra- tion Mode 2 shape will be a function of temperature, and we now want to estimate the magnitudes of the range and SWH estimation errors resulting from the on-orbit temperature changes. Figure 3-20 shows the GFO residual gain adjustments obtained

1.09 u

1.07

m t_ t .05 ¢ o m

1.03 O Im

1.01 .tt O. E 0.99 J

o 0.97 J 0+95

0.93 J

8 1_ a4 3a 40 48 s6 64 V2 80 88 96 104 _2 _20 _a8 I GFO on-board sample # (1-128)

Figure 3-20 GFO Individual Waveform Sample Gains from cai,2 with Patch, at Temperatures 30 and 45C I as the inverse of Calibration Mode 2 data from GFO thermal testing in September 1996 for two different temperatures, 30 C and 45 C. The GFO smile patch was already D loaded into the GFO altimeter for these tests.

From Launch to Acceptance Page 3-26 December 2000 Assessment of Instrument Performance GFO Altimeter Engineering Assessment Report

We can use the same type of simulation study as in the no-patch investigation in Sec- tion 3.7. This time we produced a large simulated set of GFO responses to waveforms generated for SWH from 0 to 20 m and attitude 0.0 to 0.4 degrees assuming that the waveform sample response would have been perfectly corrected by the 30C gains of Figure 3-20. This set of simulated data was used to build a table look-up correction procedure. Then we ran the simulation for a set of SWH and attitudes using the 45 C gains of Figure 3-20. We then entered these 45 C results into the 30C table look-up procedure, and were thus able to estimate the range and SWH estimation consequences of a change from 30 to 45 C.

The range correction error is plotted in Figure 3-21 for SWH 0 to 14 m and for attitude 0.2, 0.3, and 0.4 degrees. There is little attitude sensitivity in this result. The gate index transitions are readily seen in this Figure. The range estimation change is less than 3 millimeters for all SWH less than 13 m. At the gate index change from 4 to 5 at about 13 m, SWH the range estimation error suddenly increases to 7 millimeters, and increases with increasing SWH. Figure 3-21 is for a 15 degree C temperature change, from 30 C to 45 C. We expect the waveform sample shape effects (as shown by Cali- bration Mode 2) to be relatively linear with temperature, so we expect that the GFO range variations for a 5 C temperature change to be only 1/3 of the size of the Figure 3-21 range error estimate. Thus we're predicting sub-centimeter range estimation consequences of the 5 degree C variations of the GFO on-orbit temperature. L FI

8 F ==

7 -_ o Att 0.1 E • Att 0.2 E 6 Att 0.3

r, 2 5-

c o 4 • •

O 3

0 0 2 4 6 8 10 12 14 SWH, m

Figure 3-21 GFO Range Correction Difference vs. SWH Comparing 45C to 30C Cal-2 Data

Figure 3-22 shows the SWH estimation error resulting from the 30 C to 45 C temperature change and the largest SWH error shown, at 14 m SWH, is only about 0.15 m.

December 2000 Page 3-27 From Launch to Acceptance GFO Altimeter Engineering Assessment Report Assessment of Instrument Performance ,--

Therefore the GFO estimation error for SWH is expected to be within 0.05 m for a 5 w degree change in GFO temperature.

m W

0.16 i

0.14 _--",_

0.12 - ° Att 0.1

E • Att 0.2 0.to _ Att 0.3 _ ¢.

0.08 ;4, "O

0.06 _

O 0.04 o in -r- 0.02

0.00 _ ,, I -002 o

-0.04 I 0 2 4 6 8 10 12 t4

SWH, m

Figure 3-22 GFO SWH Correction Difference vs. SWH i Comparing 45C to 30C Cal-2 Data

This simulation study has shown that GFO range and SWH estimation errors for a 15 g C temperature change are much less than the errors for no-patch operation (Section 3.6), as we expect. More importantly, this study shows that normal GFO thermal variations will have no practical consequences to range and SWH estimates. No thermal I corrections were planned for GFO range and SWH, and we are pleased to have shown that no such corrections are needed.

m i u

From Launch to Acceptance Page 3-28 December 2000

If Assessment of Instrument Performance GFO Altimeter Engineering Assessment Report

3.9 Sigma0 Blooms and Examples in GFO Data

Over several years of our continuing assessment of TOPEX Altimeter performance, we have been investigating a phenomenon we designated as the sigma0 bloom. A sigma0 bloom is characterized by abnormally high sigma0 values, typically persist- ing for some tens of seconds. The occurrence of sigma0 blooms is generally correlated with regions of lower significant waveheights. The strong radar returns from the sigma0 blooms are apparently the result of very smooth ocean surfaces within the radar altimeter footprint, either as a result of very calm winds or as a consequence of surface slicks. The slick regions may be irregularly distributed within the altimeter footprint. In a sigma0 bloom region the classical radar altimeter waveform model is not valid, and the range results can be affected by the departure of the return shape from the ocean surface scattering model for which the over-ocean radar altimeter was designed. We have estimated that almost 5% of the TOPEX over-ocean data are con- taminated by sigma0 blooms, and we have found examples of blooms in data from other radar altimeters including Geosatr ERS-1, and Poseidon-1. As expected, the GFO data also have regions of sigma0 blooms.

In this section we will present a pair of typical sigma0 blooms from GFO data, and will discuss a possible scheme for identifying data regions in which blooms are present. Figure 3-23 shows the 1-second sigma0 for an approximately 13 minute portion of 2000 day 252 data. The horizontal axis of this, and all subsequent figures in the GFO bloom examples, is time in seconds relative to an arbitrary J2000 time of 21664440.497 seconds. Notice the obvious sigma0 bloom examples at about 200 and 700 seconds on the horizontal axis of Figure 3-23.

1-second NGDR averages in 2000day252 time (hh:mm) 05:54 to 06:07 26

22 • • approximate center at -21.80 degrees m._ 20 • latitude, 91.57 degrees longitude

v @ _18 - - ,_af ...... ,=

16_ " • approx{mafecenteratO.O3d?yrees !

ZO 14 ._=• e fatituoe, ,_. 7_l oegrees to giruoe _ __ _o

l_. 5L OO at 2 "P._r "t " .,,¢" ,_ L

8 0 100 200 300 400 500 600 700 800 Time, seconds, tel. to J2000 21,664,440.497

Figure 3-23 NGDR Sigma0 vs.Time

December 2000 Page 3-29 From Launch to Acceptance GFO Altimeter Engineering Assessment Report Assessment of Instrument Performance _,

Figure 3-24 shows the GFO wind speed estimates for the same time as Figure 3-23, m

l-second NGDR averages in 2000clay252 time (hh:mm) 05:54 to 06:07 m 18 m

"o 16

14__ • 12 E

"O 8

"O u e- 6

4 m (.9 Z m 2

0 0 100 200 300 400 500 600 700 BOO w Time, seconds, rel. to J2000 21,664,440.497

Figure 3-24 NGDR Wind Speed vs. time m and Figure 3-25 shows the corresponding 1-second significant waveheight estimates.

I l-second waveform averages in 2000day252 time (hh:mm) 05:54 to 06:07 5.5

o o fit SWH_m 5.0 - - • _GFO SWH_m

0 O O 0 O U_

)0 0 0 0 0 0

0D 1.5

.= L m 1.0

0 100 200 300 400 500 600 700 800 Time, seconds, rel. to J2000 21,664,440.497

Figure 3-25 GFO 1-Sec Waveform Fit SWH vs. Time U

Figure 3-25 shows both the GFO SWH estimates (available from the NGDR or the SDR) and the SWH estimates from waveform fits to 1-second averages of the GFO waveform samples.

j -

From Launch to Acceptance Page 3-30 December 2000

m m Assessment of Instrument Performance GFO Altimeter Engineering Assessment Report

_J Figure 3-26 shows the GFO attitude-related quantity Vatt for this 2000day 252 bloom

1-second NGDR averages in 2000day252 time (hh:mm) 05:54 to 06:07 1.5_

Vatt 1.4 VattFit

1.3

1.2

> 1.1 0 z 1.0

0.7 0 100 200 300 400 500 600 700 800 Time, seconds, rel. to J2000 21,664,440.497

Figure 3-26 NGDR Vatt vs.Time

region. The NGDR contains both the 1-second Vatt and the fitted Vatt which is in effect a smoothed and shifted version of the 1-second Vatt. The fitted Vatt is not particularly sensitive to the sigma0 blooms but the 1-second Vatt values do show marked increase in noise and amplitude in the regions of the two sigma0 blooms. Fig- ure 3-27 shows two different GFO attitude angle estimates: the first estimate, the NGDR Attitude, is a function of the fitted Vatt; and the second, the fit ATT, is obtained from the waveform fits to 1-second averages of the GFO waveform samples. The attitude estimates from the waveform fits show behavior similar to the 1-second Vatt. Notice that the attitude estimates from waveform fits have some negative attitude estimates. There is of course no physical meaning to a negative attitude angle, which is instead a fitting procedure artifact which signals that the plateau region of the waveform is decaying more rapidly than allowed by even a zero attitude angle for the waveform return model being used in the fitting. This just another way of say- ing that the model waveform return on which radar altimeter design is based is par- tially invalid in regions of the sigma0 blooms.

Figure 3-28 plots the NGDR quantity SSHU STD in the bloom example region. The SSHU STD is the standard deviation of the ten individual SSHU estimates within the nominal 1-second data frame of the GFO altimeter, and SSHU is the sea-surface height without any environmental corrections. Figure 3-28 shows noisier behavior in the regions of the two blooms, at 200 and 700 seconds on the time axis, but except for a few higher spikes it, s not particularly easy to see the change in noise relative to the rest of the data. To show the noise change more clearly, a 15-second time window was chosen and a standard deviation was estimated for the 15 SSHU STD values in this

December 2000 Page 3-31 From Launch to Acceptance GFO Altimeter Engineering Assessment Report Assessment of Instrument Performance ..,,

k 1-second waveform averages in 2000day252 time (hh:mm) 0.6 ......

o fit ATT_deg _ o o _NGDR Attitude _ i o i " 0 __ r9 o 0 m 0 4 -_- O r_Oo i_

= QO _ : O m

-0.2 , : O o

-0.4

= 0 m u

-0.6 0 100 200 300 400 500 600 700 800 Time, seconds, rel. to J2000 21,664,440.497 u Figure 3-27 GFO 1-Sec Waveform Fit Attitude vs. Time

I 1-second NGDR averages in 2000day252 time (hh:mm) 05:54 to 06:07 0.25

U 0.20

e 0.15 d I- I -I- 0.10

g 0.05

m 0.00 - 0 100 200 300 400 500 600 700 800 Time, seconds, rel. to J2000 21,664,440.497 u Figure 3-28 SSHU STD vs.Time for GFO Data Segment B, in 2000 Day 252

window. This 15-second window was slid through the entire set of data and the U results were plotted every (nominal) second in Figure 3-29. There is nothing special about the choice of a 15-second windov¢, and the results are not highly sensitive to the particular time width chosen. The point of this exercise was to show that there m were range estimation noise consequences of the presence of sigma0 blooms.

From Launch to Acceptance Page 3-32 December 2000 E " __ Assessment of Instrument Performance GFO Altimeter Engineering Assessment Report

1-second NGDR averaoes in 2000dav252 time fhh:mm_ 05:54 to 06:07 0.07 .....

0.06 _SSHU STD StDev i

_" 0.05

0.03

u} 0.02 =="_'_ 0.04 _

0,01

0.00 0 100 200 300 400 500 600 700 800 Time, seconds, rel. to J2000 21,664,440.497

Figure 3-29 NGDR SSHU STD Standard Deviation vs.Time

There is no reason not to expect that there would be range biases as well in presence of sigma0 blooms. In absence of a clearly characterizable waveform shape evolution through a bloom event, it is not possible to estimate how large a bloom-caused range bias would be. A very rough guess would be several centimeters. Therefore anyone trying to use radar altimeter data in problems requiring better than 10 centimeter resolution would be well advised to develop editing criteria to remove bloom-con- taminated data from those problems.

Our studies of sigma0 blooms in the TOPEX data, as well as our more limited bloom studies to date in the GFO data, indicate that the anomalously increased sigma0 estimates are always accompanied by waveform shape changes which would manifest themselves in an increase in the noise of the 1-second Vatt. There would be no particular concern about high sigma0 values if there were no accompanying waveform shape changes; it is the shape changes which lead to the possible range estimate changes. Figure 3-30 shows the standard deviation of 1-second Vatt estimates within a 15-second sliding time window; it is the same technique used for the standard deviations of SSHU STD shown in Figure 3-29.

A first guess at a bloom-editing recipe for GFO might be:

1) Edit out any data for which the 1-second sigma0 value is greater than 14 dB. (Actually in our TOPEX experience, we,re suspicious of any sigma0 greater than 13 or 13.5 dB.)

2) Form the set of 15-second sliding time window standard deviation estimates of the 1-second Vatt values (as plotted in Figure 3-30), and remove any data for which the 15-second Vatt standard deviation exceeds some threshold. Figure 3-30 suggests that 0.04 might be a reasonable threshold.

December 2000 Page 3-33 From Launch to Acceptance GFO Altimeter Engineering Assessment Report Assessment of Instrument Performance ,,--

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From Launch to Acceptance Page 3-34 December 2000 Section 4 WFF's Processing Recommendations to GFO Project

Wallops has made specific processing recommendations to the GFO Project, with respect to:

• temperature correction for AGC

• bias corrections to sigma0 and SWH

The basis for these recommendations is discussed in the following.

4.1 Temperature Correction for AGC

4.1,1 Findings

Examination of early NGDR data showed that GFO's AGC and Sigma0 were both strongly correlated with Receiver Temperature. Their slopes were both negative with respect to increasing temperature.

The WFF-calculated additive temperature-effect correction (normalized at 41.8 degrees), applicable to both NGDR AGC and NGDR Sigma0, is: w WFF Corr (NGDR) = -9.1492 dB + 0.2188 dB/degree * temperature.

4.1.2 Background

Prior to any temperature correction, AGC was observed to have a negative slope with respect to increasing temperature.

Pre-Launch TV testing showed CAL-1 AGC temperature correction (normalized at 41-8 degrees) of:

WFF Corr (TV_Cal) = -3.6191 dB + 0.0865 dB/degree * temperature

In-Flight CAL-1 AGC shows temperature effect (normalized at 41.8 degrees) of:

WFF Corr (FLT_Cal) = -5.5301 dB + 0.1323 dB/degree * temperature

The GFO-processing correction for temperature (normalized at 41.8 degrees) was:

Corr(GFO) = +3.6191 dB - 0.0865 dB/degree * temperature

4.1.3 Analysis

For our analysis, one-minute averages of AGC, Sigma0 and SWH from NGDRs between 1999030 and 1999213 were accumulated and temperature-binned (0.1 degree bins); and then the mean of each bin was computed. The mean AGC and Sigma0 were then normalized to 2.85 m SWH, based on a WFF empirical model (derived from TOPEX data) relating SWH to backscatter. Figure 4.1 shows the analysis of the temperature correction applied to the AGC and Sigma0.

December 2000 Page 4-1 From Launch to Acceptance GFO Altimeter Engineering Assessment Report WFF's Processing Recommendations to GFO Project ,.=

GFONGDR030-213 I I I I t t t t ! b t I I I I I I I I I I t I 1 t I I I t I I I I I I I I I I I I I I I I i t t I I I I I I I I t ti12

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From Launch to Acceptance Page 4-2 December 2000 I WFF's Processing Recommendations to GFO Project GFO Altimeter Engineering Assessment Report

Our conclusion is that the AGC temperature correction coefficients used during GFO processing had the wrong sign. [The GFO coefficients made the AGC slope, with respect to increasing temperature, more negative.]

4.1.4 Recommendation

The WFF recommendation, for AGC temperature correction, was to change the temperature correction coefficients:

• Change recv lk tO from +3.6191 to-5.5301

• Change recv lk tl from-0.0865 to +0.1323 • recv lk t2 would remain 0.0

4.1.5 Implementation

This recommendation was implemented in the Payload Operations Center (POC) Systems on 11 February 2000. The first POC dataset following the change was 00042/ 173349Z. The first sdr dataset following the change was 00042/134745Z.

4.2 WFF Recommended Sigma0 and SWH Corrections

Based on the various studies in section 3, WFF recommends using the sigma0 and SWH corrections that resulted from the windspeed study using NCEP winds as the reference as summarized in section 3.5 and attached as this report's Appendix A. We believe this method is our best sigma0 calibration method and have chosen to keep the matching SWH calibration as our recommendation. This analysis determined that the GFO sigma0 needs to be increased by 0.37 dB and the SWH needs to be increased about 0.24 meters.

December 2000 Page 4-3 From Launch to Acceptance GFO Altimeter Engineering Assessment Report WFF's Processing Recommendations to GFO Project

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From Launch to Acceptance Page 4-4 December 2000

m Section 5 Engineering Assessment Synopsis

5.1 Performance Overview

Our analyses of the GFO altimeter demonstrate that it is performing well. Its range measurement precision is comparable with contemporaneous satellite radar altimeters, including TOPEX. Its internal calibrations and its cycle-to-cycle global averages have been very consistent. Comparisons with other sensors indicate that measurement biases are within GFO's pre-flight specifications of: SWH +/- 0.5m, sigma0 +/-01 dB, and windspeed +/- 2 m/s.

During the assessment of the GFO altimeter performance, WFF encountered a number of data problems that were the results of the errors in the ground data processing. Considerable time was spent in the analysis of each problem and the communication of these back to the GFO project. These problems made our performance analysis dif- ficult and often were wrongly associated with the altimeter performance. The major ground software problems have been fixed, but several ground processing software changes that will improve the data integrity remain to be implemented. Some fine- tuning of GFO altimeter data correction algorithms and data processing algorithms have been shown to be necessar_ and most have been implemented. However, fine- tuning has also occurred for every satellite radar mission flown to date.

We are continuing our GFO altimeter performance assessment on a daily basis, and are continuing to develop improved analysis techniques. Supplemental performance reports will be issued on a regular basis, and special reports will be prepared as warranted.

5.2 Open Issues

There are some remaining open issues regarding our assessment of the performance of the GFO altimeter. Those issues are:

• Definitive study of land-to-water acquisition times • SWH Bias

Some of the remaining issues regarding data integrity are:

• Time-tag assignment to bad data caused by bad down link

- SDR/NGDR occasional data misalignment- Example: Day 2000150

- Erroneous datasets are occasionally received for future dates - Example: Day 2003246

• Occasional tropospheric correction problem at 0 degree longitude

• NGDR quality assurance - Example: Day 2000147

December 2000 Page 5-1 From Launch to Acceptance = GFO Altim.eter Engineering Assessment Report Engineering Assessment Synopsis ,,,,,

Data sets are received that are processed with a constant composite receiver U temperature. This causes data correction errors.

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From Launch to Acceptance Page 5-2 December 2000 m m Section 6 References

6.1 Supporting Documentation

D.W. Hancock III, 1995, GFO Altimeter Height Noise Comparison with TOPEX, NASA/WFF report, http://gfo.wff.nasa.gov/docs.html

D.W. Hancock III, et al, 1998, On the Evaluation of the GEOSAT Follow-on (GFO) Altimeter. NASA/WFF presentation at the AGU meeting in San Francisco, CA, on December 10, 1998. http://gfo.wff.nasa.gov/docs.html

G.S. Hayne, 1996, Geosat Follow-On Altimeter Height Loop and AGC Loop Step Responses from Ground Testing. NASA/WFF report, http://gfo.wff.nasa.gov/ docs.html

G.S. Hayne and D.W. Hancock III, 2000, GFO Radar Altimeter Performance. NASA/ WFF GFO presentation at GFO meeting in Washington, D.C., on July 20, 2000. http:/ /gfo.wff.nasa.gov/docs.html

G.S. Hayne and D.W. Hancock III, 1998, GFO Preliminary Results for Waveform Fit- ting to On-Orbit Waveform Sample Data. NASA/WFF report, http:// gfo.wff.nasa.gov/docs.html

J.E. Lee, Documentation for the GFO File Transfer System. NASA/WFF report, http:/ /gfo.wff.nasa.gov/docs.html

Naval Oceanographic Office, 2000, Navy-IGDR Users Handbook. http://gfo.bmpcoe. org / G fo / Data_val / Ca l_form ats / formats, htm

Naval Oceanographic Office, 1998, SDR Format, Contents and Algorithms. http:// gfo.bmpcoe .org / Gfo / Data_val / Cal_formats / formats .htm

Naval Oceanographic Office, 1998, NGDR Format, Contents and Corrections. http:// gfo.bmpcoe.org/Gfo/Data_val/Cal_formats/formats.htm

N. Tran, D.W. Hancock III, G.S. Hayne, D.W. Lockwood and D. Vandemark, 2000, GFO Altimeter Sigma0 and SWH Calibration Correction. NASA/WFF report.

December 2000 Page 6-1 From Launch to Acceptance GFO Altimeter Engineering Assessment Report References --

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From Launch to Acceptance Page 6-2 December 2000 u m ,, m Appendix A

GFO altimeter sigma-naught and swh calibration correction

N. Tran, D. W. Hancock III, G. S. Hayne, D. W. Lockwood and D. Vandemark

NASA Goddard Space Flight Center, Wallops Flight Facility, Wallops Island, Virginia

DRAFT - 8 December 2000

Abstract A comparison study has been carried out between GEOSAT Follow-on (GFO) and TOPEX altimeters for the radar cross section (sigma-naught) and the significant wave height (swh) by using National Centers for Environmental Prediction (NCEP) wind. This study shows that the GFO cross section values are biased about 0.37 dB lower than the TOPEX cross section, and the GFO significant wave height values are biased about 0.24 m lower than the TOPEX significant wave height. This paper describes how these GFO calibration corrections were generated and recommends these additive contributions to both GFO cross section and significant wave height to improve the wind speed retrieval. The correction on GFO sigma- naught leads to statistically significant global error reductions for the GFO wind speed. The overall bias between GFO and NCEP wind speeds decreases from 1.75 m to 0.53 m s-1 and the root-mean-square (rms) error from 2.44 to 1.59 m s -1 with the operational algorithm.

1 Introduction

The GEOSAT Follow-on (GFO) program is a United States Navy project. It was launched on l0 February 1998. This paper presents some results as part of the calibration and validation pro jet for GFO. Data products examined include radar cross section, significant wave height and surface wind speed. Comparison of GFO and TOPEX altimeter measurements is intended to check the proper functioning of GFO instrument.

2 Methods

Because the number of crossovers between GFO and TOPEX altimeters would be too limited to draw significant features during the period of 17 days from 3 May 2000 to 19 May 2000, we chose to compare these two sensors through the use of collocations with the National Centers for Environmental Prediction (NCEP) wind. In order to avoid problems linked to low and high wind speed, we select data between q-- 1 standard deviation from the mean of the NCEP wind speed. These subsets contain 69.55% and 69.30% of data from the global sets GFO/NCEP and TOPEX/NCEP respectively. The minimum value of wind speed is about 4.8-4.9 m s-1 and the maximum value is about 11.0-11.2 m s-1. These subsets are used to determine biases between the two altimeters and we evaluated the relevancy 2._ of these corrections on GFO measurements by comparing respectively GFO and TOPEX wind speeds with NCEP winds over the whole wind speed range. We used in both cases their operational modified Chelton and Wentz (MCW) algorithms [1986] on the I0 second average radar cross sections to compute the altimeter wind speed. J 3 Data

GFO data are from NGDR files and TOPEX data are from GDR during the same period of time of 17 days W from 3 May 2000 to 19 May 2000. We applied TOPEX Side B sigma-naught calibration Table adjustements [Hayne and Hancock III, 2000] to the TOPEX GDR cross section. The different measurements used are a 10 U second average. These altimeter data are limited in space between 60°N and 60°S. The NCEP model provides near-surface wind vector estimates for an altitude of l0 meters above the = ground and for neutral stability. Their output is on a 1.0 x 1.0 degree grid every 6 hours. For our purpose we i interpolated the model outputs in space and time to derive a wind estimate coiocated with GFO (or TOPEX) observations,

After completing comparisons between altimeter wind speeds and NCEP winds over the entire wind Gig speed range, we repeated the TOPEX/GFO radar cross section and significant wave height comparisons for two later 17-day periods to demonstrate consistency of the biases found in the 3-19 May 2000 dataset. m

4 Results m II For the subsets described previously the comparisons show that the GFO cross section values are biased about 0.37 dB lower than the TOPEX cross section, and the GFO significant wave height values are biased about 0.24 m lower than the TOPEX significant wave height. U

Days GFO TOPEX TOPEX-GFO m < o'0 > (dB) 124-140 10.86 11.23 0.37 J < swh > (m) 124-140 2.33 2.57 0.24

Table 1" Mean values for GFO and TOPEX radar cross section and significant wave height on the respective i subset for year 2000, days 124-140.

i We define the wind speed difference at l0 meters, T_ _,-,.:

(1)

Winds are in m s -1,and the wind error standard deviation is simply:

std = (< _,_,,r2 > - < L_,, >2)2 (2) I

This factor will be of some value in providing a relative algorithm assessments but is obviously equivalent to the rms when the bias nears zero. Denoting the wind error e, a s_h is defined as the slope in the linear regression e = a:s_,h swh + _. This slope is checked because numerous past studies have suggested that long scale gravity waves, which do not adjust to wind as quickly as the shor t gravity-capillary waves, have m U a measurable influence on interpretation of altimeter returns in term of wind speed. Most studies appear to agree that the effect is clearly of second order and that swh serves as a limited proxy for removing sea-state impacts from the ahimeter wind speed estimate, in our case, we used such a parameter to valid our proposed W corrections both on radar cross section and significant wave height. Tables 2 to 5 present some statistical comparisons between altimeter wind speeds and NCEP winds over the whole wind speed range: Table 2 for GFO Wind speed retrieved by using GFO MCW algorithm as on m NGDR without correction on cross section and Table 5 with correction of 0.3725 dB on cross section before

i computing the new GFO wind speed. For comparison purposes, Table 3 presents the same statistical parameters about the comparison between TOPEX and NCEP wind speeds. In Table 4 we used a new TOPEX algorithm, described in [Gourrion et al., 2000], on the GFO measurements in order to take into account both corrections on cross section and on significant wave height. This new model relates TOPEX altimeter measurements of radar cross section and significant wave height to near-surface wind speed.

Nb Nb < UGFO > < UNCEP > bias std rms % error e_swh total after filter (m/s) (m/s) < _%_r > (m/s) (m/s) > 2m/s 91215 66389 9.58 7.84 1.75 !.71 2.44 51.79 0.31

Table 2: Statistical parameters of the comparison between GFO and NCEP wind speeds by using GFO MCW algorithm without correction on (_r°) on the whole wind speed range for year 2000, days 124-140.

Nb Nb < UTPX > < UNCEP > bias std rms % error c_s_h total after filter (m/s) (m/s) < /_r > (m/s) (m/s) > 2m/s 101371 70475 8.43 8.02 0.41 1.47 1.53 18.89 0.39

Table 3: Statistical parameters of the comparison between TOPEX and NCEP wind speeds by using TOPEX MCW algorithm on the whole wind speed range for year 2000, days 124-140.

Nb Nb < UGFO > < UNCEP > bias std rms % error a,_h total after filter (m/s) (m/s) T (m/s) (m/s) >2m/s 91215 66466 7.80 7.83 -0.04 1.47 1.47 16.85 0.06

Table 4: Statistical parameters of the comparison between GFO and NCEP wind speeds by using a new TOPEX algorithm [Gourrion et al.,2000], Ulo = f(_ro, swh), with correction on (swh_or = swh + 0.2381) and on (ac°_ = _r° + 0.3725) on the whole wind speed range for year 2000, days 124-140.

We note that the correction on GFO sigma-naught leads to statistically-significant global error reductions for the GFO wind speed. The overall bias between GFO and NCEP wind speed decreases from 1.75 m to 0.53 m s-1 and the root-mean-square (rms) error from 2.44 to 1.59 s -1 with the operational algorithm. The percentage of data for which the difference between GFO and NCEP wind speeds are greater than 2 m s-1 decreases more than half. As an additional note, when we computed new GFO winds by using the new TOPEX algorithm [Gourrion et al.,2000] with correction both on sigma-naught and swh the values of the overall bias and of the slope _h are close to zero. Figure 1 presents the comparison between GFO wind speeds without correction and NCEP winds with respect to the wind speed. Between 7 and 15 m s -1 the difference between the two sources is in average = greater than 2 m s -1. Figure 2 presents the same results when we used the correction on cross section to compute a new GFO wind. We can note that when we take into account the correction on cross section, the new GFO wind speeds appear to be more coherent with NCEP over the whole wind speed range. This second scatter diagram shows that the data are closer to the perfect line and the difference between these new GFO wind speed values and those of NCEP are in average almost between + 1 m s -1. Figure 2 is similar to Figure 3 which presents the comparison between TOPEX wind speeds and NCEP winds with respect to the wind speed. J

Nb Nb < Uayo > < Umc_e > bias std rrns % error as_h w total after filter (m/s) (m/s) < U_rr > (m/s) (m/s) > 2m/s 91215 66362 8.37 7.84 0.53 1.50 1.59 20.50 0.28

i Table 5: Statistical parameters of the comparison between GFO and NCEP wind speeds by using GFO MCW algorithm with correction on (o's°or = cr° + 0.3725) on the whole wind speed range for year 2000, days 124- 140.

GFO Start date: 124, End date: 140, Number of data: 66389 GFO Start date: 124, End date: 140, Number of data: 66389 30 I ! I i i : i I I I I I t I 28 Withoutc0rrecti0n :. i .-i-..i-..-i...,_ i/ 3" without c0rrecti0n 26 UGF O = !,!4 UNCEP + 0..6_, : t S /. R 2 =0_919: !" ! "? : i t • "/ " _24 "C_ i " ' i ' ; " ? Z " " E22 m -_ 20

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GFO Start date: 124, End date: 140, Number of data: 66389 GFO Start date: 124, End date: 140, Number of data: 66389 m

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Figure 1: Comparison between GFO winds by the GFO MCW algorithm without correction and NCEP winds with respect to NCEP wind speed.

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m GFO Start date: 124, End date: 140, Number of data: 66362 GFO Start date: 124, End date: 140, Number of data: 66362 30 q , , , , , , • ', _ , , , , f, I I I ! ! I ! ! I I I I I I 28 with correction : : : : : ...... _,/ with correction ...... : ..:- :-:-- --:-:- I(/3 26 :U(3FO =...... 1.1 UNCEP ÷ -0.244 : ot/ E _2, R2=o1929. i : i..i ..:.//, . v (/) ..... : :# i . i L _ . i i i . el. -.:-. T.-.-:.-! -.:-.-. -.-- -i.-.:-. T. T.,-_--- -o 2o ""

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GFO Start date: 124, End date: 140, Number of data: 66362 GFO Start date: 124, End date: 140, Number of data: 66362 30 ...... 5 :,,..:/ 28- withcorrectton : ...... :>' / : with correction 26 ...... i....i ..i..i . .... :-Y / / "" •

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2 0 2 4 6 8 10 12 14 16 1B 20 22 24 26 28 30 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 NCEP wind speed (ms -_) NCEP wind speed (m s -1)

Figure 2: Comparison between GFO winds by the GFO MCW algorithm with correction on radar cross section and NCEP winds with respect to NCEP wind speed.

Days GFO TOPEX TOPEX-GFO < O-o > (dB) 245-260 10.95 !1.32 0.37 (244 missing) 261-277 i 0.97 11.33 0.36 < swh > (m) 245-260 2.16 2.38 0.22 (244 missing) 261-277 2.24 2.49 0.25

Table 6: Mean values for GFO and TOPEX radar cross section and significant wave height on the respective

= subset for year 2000, days 245-277. I

TOPEX Start date: 124, End date: 140, Number of data: 70475 TOPEX Start date: 124, End date: 140, Number of data: 70475 30 u 28

_26 UTOPE X =._l..13 UNCEP -0,63. _ #.#/

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TOPEX Start date: 124, End date: 140, Number of data: 70475 TOPEX Start date: 124, End date: 140, Number of data: 70475 5 ! ......

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Figure 3: Comparison between TOPEX winds by the TOPEX MCW algorithm and NCEP winds with respect to NCEP wind speed. I

[ J

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0 GFO (MCW) w/o. corr. wind speed (m ,-1)

Figure 4: Comparison between GFO wind speed retrieval by using GFO MCW algorithm with and without correction on radar cross section

Figure 4 presents a comparison between GFO wind speed retrieval by using GFO MCW algorithm with and without correction on radar cross section with respect to GFO wind without correction. The improvement appears to be for all wind speeds. Above 5 m s -1 , the difference between these two GFO wind speeds is close to l ms -1. To evaluate the consistency of the biases on sigma-naught and on SWH pointed out in Table 1, we used two other periods of !7 days from 31 August 2000 to 3 October 2000. The results are presented in Table 6 and the agreement of the TOPEX/GFO differences in Table 1 and Table 6 confirms the values of the biases found.

5 Conclusion

We have determined that the GFO altimeter calibration bias to be added to SWH is 0.24 m. This will be a constant shift from the current SWH values that we feel needs to be applied based using TOPEX as ihe standard.

= The calibration bias to be added to sigma-naught is 0.37 dB; this implies a variable wind speed correction. Using GFO MCW wind speed algorithm the effects on the estimated wind speed on the GFO products is shown in the previous Figure 4. The maximum error is less than 2 m s -1 . Both of these calibrations are less than the specification ( swh + 0.5 m, sigma-naught 4- 1 dB, wind speed 4- 2 m s -1) but we recommend that they be implemented in the standard GFO data processing.

6 References

Chelton D. B. and Wentz F. J., Further development of an improved altimeter wind-speed algorithm, J. of Geophys. Res., 91, C12, 14250-14260, 1986.

Gourrion J., Vandemark D., Bailey S., Chapron B., Satellite altimeter models for surface wind speed developed using ocean satellite crossovers, IFREMER Tech. Report, DRO/OS-00/01,2000.

Hayne G. S. and Hancock llI D. W., TOPEX Side B sigma0 calibration table adjustements, WFF TOPEX documents, November 2000. Im

m II

m lie

lip

uw _

z Abbreviations & Acronyms

CAL Calibration Mode or Calibration Mode data

Cal/Val Calibration and Validation

CPU Central Processing Unit

EDAC Error Detection and Correction Circuits

EEPROM Electrically Erasable Programmable Read Only Memory

ENG Engineering Data

ERO Exact Repeat Orbit

FTP File Transfer Protocol

GEOSAT Geodetic Satellite

GFO GEOSAT Follow-On

GPSR Global Positioning Satellite Receiver

GSFC Goddard Space Flight Center

HW Hardware

IAP Integrated Avionics Processor

IDL Interactive Data Language

NCEP National Centers for Environmental Prediction

NGDR NOAA Geophysical Data Record r m NSI NASA Science Internet

OODD Operational Orbit Determination Data

POC Payload Operations Center

QSCAT NASA QuikSCAT satellite

RA Radar Altimeter

RAM Read Access Memory

RASE Radar Altimeter System Evaluator

SCI Science Data

SDR Science Data Record

SDT Science Definition Team

SMA Semi-Major Axis of the orbit

SW Software

December 2000 Page AB-1 From Launch to Acceptance GFO Altimeter Engineering Assessment Report Abbreviations & Acronyms ,,,-

UTC Universal Time Code g VTCW Vehicle Time Code Word

Bm WF Waveform Data m

WFF Wallops Flight Facility

WVR Water Vapor Radiometer g

m I

From Launch to Acceptance Page AB-2 December 2000 I P Wh.,v

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L REPORT DOCUMENTATION PAGE FormApproved OMB No. 0704-0188

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructiOnS, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this correction of information, including suggestions for reducing this burden, to Washington Headquarters Services. Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 4204. Arlington, VA 22202-4302. and to the Office of Management and Budget, Paperwork Reduction Project (0704-0188), Washington, DC 20503. I 1. AGENCY USE ONLY (Leave b/ank) 2. REPORT DATE 3. REPORT TYPE AND DATESCOVERED March 2001 Technical Memorandum m

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS IB GEOSAT Follow-on (GFO) Altimeter Document Series Vol. 1: GFO Altimeter Engineering Assessment Report, Version 1 Code 972 NAS5-00181 6. AUTHOR(S) 972-622-47-36-78 D.W. Hancock III, G.S. Hayne, R.L. Brooks, and D.W, Lockwood I

7, PERFORMING ORGANIZATION NAME(S) AND ADDRESS (ES) 8. PEFORMING ORGANIZATION REPORT NUMBER Observational Science Branch Laboratory for Hydrospheric Processes u GSFC Wallops Flight Facility 2001-02001-0 Wallops Island, Virginia 23337 i u 9. SPONSORING / MONITORING AGENCY NAME(S) AND ADDRESS (ES) 10. SPONSORING / MONITORING AGENCY REPORT NUMBER National Aeronautics and Space Administration TM-200 i -209984/Ver 1/Vol. l m Washington, DC 20546-0001 []

11. SUPPLEMENTARY NOTES z R.L. Brooks and D.W. Lockwood: Raytheon ITSS. gl

12a. DISTRIBUTION / AVAILABILITY STATEMENT 12b. DISTRIBUTION CODE Unclassified-Unlimited Subject Category: 61 Report available from the NASA Center for AeroSpace Information, 7121 Standard Drive, Hanover, MD 21076-1320. (301) 621-0390. g _. i i 13. ABSTRACT (Maximum 200 words) BII The U.S. Navy's Geosat Follow-On (GFO) Mission, launched on February 10, 1998, is the latest in a series of altimetric satellites which inch=de Seasat, Geosat, ERS-1, and TOPEX]POSEIDON (T/P). The purpose of this

report is to document the GFO altimeter performance determined from the analyses and results performed by u the NASA/GSFC/Wallops altimeter calibration team. It is the first of an anticipated series of NASA/GSFC/ Wallops' GFO performance documents, each of which will update assessment results. This report covers the performance from launch to instrument acceptance by the Navy on November 29, 2000. Data derived from GFO will lead to improvements in the knowledge of ocean circulation, ice sheet topography, and climate change. In order to capture the maximum amount of information from the GFO data, accurate altimeter calibrations are required for the civilian data set which NOAA will produce. Wallops Flight Facility has provided III similar products for the Geosat and T/P missions and is doing the same for GFO.

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14. SUBJECT TERMS 15. NUMBER OFPAGES II + 86 = Geosat Follow-On, Satellite Radar Altimeter, Performance Assessment, 16. PRICE CODE Sea Surface Topography • + 171 SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION 20. LIMITATION OF ABSTRACT OF REPORT OF THIS PAGE OF ABSTRACT Unclassified Unclassified Unclassified UL

NSN 7540-01-280-5500 St"andard Form 298 (Rev. 2-89) Prescribed by ANSI Std Z39.18 298-102